implement ellipse

.vscode/settings.json

@@ -0,0 +1,5 @@
+{
+    "editor.formatOnSave": true,
+    "editor.formatOnType": true,
+    "vscode-neovim.neovimExecutablePaths.linux": "/usr/bin/nvim"
+}

learnin.csproj

@@ -2,4 +2,7 @@
   <PropertyGroup>
     <TargetFramework>net472</TargetFramework>
   </PropertyGroup>
+  <ItemGroup>
+    <PackageReference Include="vim.math3d" Version="1.6.0" />
+  </ItemGroup>
 </Project>

scenes/Main.tscn

@@ -1,18 +1,13 @@
-[gd_scene load_steps=4 format=2]
+[gd_scene load_steps=3 format=2]
 
-[ext_resource path="res://scenes/Train.tscn" type="PackedScene" id=1]
-[ext_resource path="res://scenes/Railway.tscn" type="PackedScene" id=2]
 [ext_resource path="res://scenes/OrbitCamera.tscn" type="PackedScene" id=3]
+[ext_resource path="res://scenes/OrbitSystem.tscn" type="PackedScene" id=4]
 
 [node name="Main" type="Node2D"]
 
-[node name="Train" parent="." instance=ExtResource( 1 )]
-
 [node name="DirectionalLight" type="DirectionalLight" parent="."]
 transform = Transform( 0.515898, 0.606099, -0.605386, -0.393123, 0.795389, 0.461314, 0.76112, -2.26831e-08, 0.648611, 0, 0, 0 )
 
 [node name="OrbitCamera" parent="." instance=ExtResource( 3 )]
-_speed = 1.0
 
-[node name="Railway" parent="." instance=ExtResource( 2 )]
-script = null
+[node name="Orbit System" parent="." instance=ExtResource( 4 )]

scenes/OrbitCamera.tscn

@@ -6,4 +6,5 @@
 script = ExtResource( 1 )
 
 [node name="Camera" type="Camera" parent="."]
-transform = Transform( 0.636065, -0.249781, 0.73009, 0, 0.946159, 0.323703, -0.771636, -0.205896, 0.601818, 8.49012, 5.30069, 6.71091 )
+transform = Transform( 1, 0, 0, 0, 0.964146, 0.265371, 0, -0.265371, 0.964146, 0, 2.21115, 9.48434 )
+fov = 25.0

scenes/OrbitSystem.tscn

@@ -0,0 +1,15 @@
+[gd_scene load_steps=2 format=2]
+
+[ext_resource path="res://scripts/orbits/OrbitSystem.cs" type="Script" id=1]
+
+[node name="Orbit System" type="Spatial"]
+script = ExtResource( 1 )
+_a = NodePath("A")
+_b = NodePath("B")
+_barycenter = NodePath("Barycenter")
+
+[node name="A" type="Spatial" parent="."]
+
+[node name="B" type="Spatial" parent="."]
+
+[node name="Barycenter" type="Spatial" parent="."]

scenes/RailNode.tscn

@@ -1,22 +0,0 @@
-[gd_scene load_steps=2 format=2]
-
-[sub_resource type="CSharpScript" id=1]
-script/source = "using Godot;
-using System;
-
-public class RailNode : Spatial
-{
-    public override void _Ready()
-    {
-
-    }
-
-//  public override void _Process(float delta)
-//  {
-//
-//  }
-}
-"
-
-[node name="Spatial" type="Spatial"]
-script = SubResource( 1 )

scenes/Railway.tscn

@@ -1,23 +0,0 @@
-[gd_scene load_steps=4 format=2]
-
-[ext_resource path="res://scripts/Railway.cs" type="Script" id=1]
-
-[sub_resource type="Curve3D" id=1]
-_data = {
-"points": PoolVector3Array( 0, 0, 0, 0, 0, 0, 8.10688, -1.81506, 7.57791, 0, 0, 0, 0, 0, 0, -4.46766, -3.67929, 0.175438, 0, 0, 0, 0, 0, 0, -6.79476, 2.00362, -6.79993, 0, 0, 0, 0, 0, 0, 5.56111, 6.33345, -1.84912, 0, 0, 0, 0, 0, 0, 11.2517, 3.50708, 4.91382, 0, 0, 0, 0, 0, 0, 14.9168, -0.251479, 11.0718 ),
-"tilts": PoolRealArray( 0, 0, 0, 0, 0, 0 )
-}
-
-[sub_resource type="SpatialMaterial" id=2]
-flags_unshaded = true
-vertex_color_use_as_albedo = true
-
-[node name="Railway" type="Path"]
-curve = SubResource( 1 )
-script = ExtResource( 1 )
-_color = Color( 0, 1, 0.133333, 1 )
-
-[node name="ImmediateGeometry" type="ImmediateGeometry" parent="."]
-material_override = SubResource( 2 )
-
-[connection signal="curve_changed" from="." to="." method="OnPathCurveChanged"]

scenes/Train.tscn

@@ -1,12 +0,0 @@
-[gd_scene load_steps=2 format=2]
-
-[ext_resource path="res://scripts/Train.cs" type="Script" id=1]
-
-[node name="Train" type="Node"]
-script = ExtResource( 1 )
-
-[node name="CSGBox" type="CSGBox" parent="."]
-transform = Transform( 1, 0, 0, 0, 1, 0, 0, 0, 1, 0, 0.5, 0 )
-width = 1.0
-height = 1.0
-depth = 4.0

scenes/trains/Bogie.tscn

@@ -0,0 +1,9 @@
+[gd_scene format=2]
+
+[node name="Bogey" type="Spatial"]
+
+[node name="CSGBox" type="CSGBox" parent="."]
+transform = Transform( 1, 0, 0, 0, 1, 0, 0, 0, 1, 0, 0.2, 0 )
+width = 0.9
+height = 0.4
+depth = 1.1

scenes/trains/Rail.tscn

@@ -0,0 +1,25 @@
+[gd_scene load_steps=2 format=2]
+
+[sub_resource type="Curve3D" id=1]
+_data = {
+"points": PoolVector3Array(  ),
+"tilts": PoolRealArray(  )
+}
+
+[node name="Rail" type="Spatial"]
+
+[node name="Path" type="Path" parent="."]
+curve = SubResource( 1 )
+
+[node name="CSGPolygon" type="CSGPolygon" parent="."]
+polygon = PoolVector2Array( -0.383256, -0.00220253, -0.383256, 1.0066, 0.383256, 1.00219, 0.383256, -0.00220253 )
+mode = 2
+path_node = NodePath("../Path")
+path_interval_type = 0
+path_interval = 1.0
+path_simplify_angle = 0.0
+path_rotation = 2
+path_local = false
+path_continuous_u = true
+path_u_distance = 1.0
+path_joined = false

scenes/trains/Railway.tscn

@@ -0,0 +1,47 @@
+[gd_scene load_steps=3 format=2]
+
+[ext_resource path="res://scripts/trains/Railway.cs" type="Script" id=1]
+
+[sub_resource type="Curve3D" id=1]
+bake_interval = 0.01
+_data = {
+"points": PoolVector3Array( -2.68413, 0, -1.21546, 2.68413, 0, 1.21546, -20.005, 0, -8.27961, -0.202576, 0, -2.12705, 0.202576, 0, 2.12705, -15.5484, 0.00448608, -2.35426, -1.06352, 0, -2.88671, 1.06352, 0, 2.88671, -15.8992, 0.000904083, 5.83998, -3.24122, 0, 0.202576, 3.24122, 0, -0.202576, -8.81272, 0, 10.4587, -0.468547, 0, 2.85805, 0.468547, 0, -2.85805, -3.58364, -0.000244141, 4.56199, -1.16602, 4.07454e-10, 3.45615, 1.16602, -4.07454e-10, -3.45615, -5.10296, -0.000259399, -3.5917, -2.78542, 0, -1.56996, 2.78542, 0, 1.56996, 3.2912, -0.000259399, -6.30449, -3.70976, 0, -1.39605, 3.70976, 0, 1.39605, 9.68509, -0.000259399, -0.0972629, 0, 0, 0, 0, 0, 0, 19.9658, -0.000259399, -1.11014 ),
+"tilts": PoolRealArray( 0, 0, 0, 0, 0, 0, 0, 0, 0 )
+}
+
+[node name="Railway" type="Spatial"]
+script = ExtResource( 1 )
+RailWidth = 0.05
+RailHeight = 0.1
+RailGauge = 0.8
+_railPaths = [ NodePath("L"), NodePath("R") ]
+_centreLine = NodePath("Centreline")
+
+[node name="Centreline" type="Path" parent="."]
+curve = SubResource( 1 )
+
+[node name="L" type="CSGPolygon" parent="."]
+polygon = PoolVector2Array( -0.45, 0, -0.45, 0.1, -0.35, 0.1, -0.35, 0 )
+mode = 2
+path_node = NodePath("../Centreline")
+path_interval_type = 0
+path_interval = 1.0
+path_simplify_angle = 0.0
+path_rotation = 2
+path_local = false
+path_continuous_u = true
+path_u_distance = 1.0
+path_joined = false
+
+[node name="R" type="CSGPolygon" parent="."]
+polygon = PoolVector2Array( 0.35, 0, 0.35, 0.1, 0.45, 0.1, 0.45, 0 )
+mode = 2
+path_node = NodePath("../Centreline")
+path_interval_type = 0
+path_interval = 1.0
+path_simplify_angle = 0.0
+path_rotation = 2
+path_local = false
+path_continuous_u = true
+path_u_distance = 1.0
+path_joined = false

scenes/trains/Train.tscn

@@ -0,0 +1,10 @@
+[gd_scene load_steps=3 format=2]
+
+[ext_resource path="res://scripts/trains/Train.cs" type="Script" id=1]
+[ext_resource path="res://scenes/trains/TrainCar.tscn" type="PackedScene" id=2]
+
+[node name="Train" type="Spatial"]
+script = ExtResource( 1 )
+_carPath = NodePath("Train Car")
+
+[node name="Train Car" parent="." instance=ExtResource( 2 )]

scenes/trains/TrainCar.tscn

@@ -0,0 +1,22 @@
+[gd_scene load_steps=3 format=2]
+
+[ext_resource path="res://scripts/trains/TrainCar.cs" type="Script" id=1]
+[ext_resource path="res://scenes/trains/Bogie.tscn" type="PackedScene" id=2]
+
+[node name="Train Car" type="Spatial"]
+script = ExtResource( 1 )
+Wheelbase = 5.0
+_boxNode = NodePath("CSGBox")
+_bogiePaths = [ NodePath("Bogie"), NodePath("Bogie2") ]
+
+[node name="CSGBox" type="CSGBox" parent="."]
+transform = Transform( 1, 0, 0, 0, 1, 0, 0, 0, 1, 0, 1, 0 )
+width = 1.0
+height = 1.0
+depth = 8.0
+
+[node name="Bogie" parent="." instance=ExtResource( 2 )]
+transform = Transform( 1, 0, 0, 0, 1, 0, 0, 0, 1, 0, 0, -2.5 )
+
+[node name="Bogie2" parent="." instance=ExtResource( 2 )]
+transform = Transform( 1, 0, 0, 0, 1, 0, 0, 0, 1, 0, 0, 2.5 )

scripts/OrbitCamera.cs

@@ -4,15 +4,44 @@ using System;
 public class OrbitCamera : Spatial
 {
     [Export]
-    private float _speed;
+    private float _sensitivity = 200f;
 
-    public override void _Ready()
+    private Vector2 _rotation;
+    private bool _canRotate = false;
+
+    public override void _Input(InputEvent e)
     {
-
+        if (e is InputEventMouseMotion mouseMotion)
+        {
+            HandleMouseMovement(mouseMotion);
+        }
+        if (e is InputEventMouseButton mouseButton)
+        {
+            HandleMouseButton(mouseButton);
+        }
     }
 
     public override void _Process(float delta)
     {
-        Rotate(Vector3.Up, _speed * delta);
+        Rotation = Vector3.Zero;
+        var sensitivity = -1f / _sensitivity;
+        Rotate(Vector3.Right, _rotation.y * sensitivity);
+        Rotate(Vector3.Up, _rotation.x * sensitivity);
+    }
+
+    // left click to drag
+    private void HandleMouseButton(InputEventMouseButton mouseButton)
+    {
+        if (mouseButton.ButtonIndex != (int)ButtonList.Left) return;
+
+        _canRotate = mouseButton.Pressed;
+    }
+
+    private void HandleMouseMovement(InputEventMouseMotion mouseMotion)
+    {
+        if (!_canRotate) return;
+
+        var delta = mouseMotion.Relative;
+        _rotation += delta;
     }
 }

scripts/Railway.cs

@@ -1,53 +0,0 @@
-using Godot;
-using System;
-
-[Tool]
-public class Railway : Path
-{
-    [Export]
-    private Color _color;
-    
-    [Export]
-    private float _gauge = 1f;
-    
-    private ImmediateGeometry _geo = null;
-    private Vector3[] _points = null;
-    
-    public void OnPathCurveChanged() 
-    {
-        Curve.Tessellate();
-        _points = Curve.GetBakedPoints();
-    }
-    
-    // Called when the node enters the scene tree for the first time.
-    public override void _Ready()
-    {
-        base._Ready();
-        
-        _geo = GetNode<ImmediateGeometry>("ImmediateGeometry");
-        
-        var m = new SpatialMaterial();
-        m.VertexColorUseAsAlbedo = true;
-        m.FlagsUnshaded = true;
-        _geo.MaterialOverride = m;
-
-        OnPathCurveChanged();
-    }
-
-    public override void _Process(float delta)
-    {
-        base._Process(delta);
-        
-        if (_geo == null) return;
-        if (_points == null || _points.Length == 0) return;
-        
-        _geo.Clear();
-        _geo.Begin(Mesh.PrimitiveType.LineStrip);
-        _geo.SetColor(new Color(1, 0, 0));
-        for(int i = 0; i < _points.Length; i++)
-        {
-            _geo.AddVertex(_points[i]);
-        }
-        _geo.End();
-    }
-}

scripts/Tracks.cs

@@ -1,22 +0,0 @@
-using Godot;
-using System;
-
-[Tool]
-public class Tracks : ImmediateGeometry
-{
-    private SpatialMaterial _m;
-
-    public override void _Ready()
-    {
-        // set up a material for imgui to use 
-        _m = new SpatialMaterial();
-        _m.VertexColorUseAsAlbedo = true;
-        _m.FlagsUnshaded = true;
-        this.MaterialOverride = _m;
-    }
-
-    public override void _Process(float delta)
-    {
-        GD.Print("hi!");
-    }
-}

scripts/Train.cs

@@ -1,16 +0,0 @@
-using Godot;
-using System;
-
-public class Train : Node
-{
-    // Called when the node enters the scene tree for the first time.
-    public override void _Ready()
-    {
-    }
-
-//  // Called every frame. 'delta' is the elapsed time since the previous frame.
-//  public override void _Process(float delta)
-//  {
-//      
-//  }
-}

scripts/orbits/ILocation.cs

@@ -0,0 +1,7 @@
+using Godot;
+using System;
+
+public interface ILocation
+{
+    Vector3 Position { get; }
+}

scripts/orbits/IMassive.cs

@@ -0,0 +1,7 @@
+using Godot;
+using System;
+
+public interface IMassive
+{
+    float Mass { get; }
+}

scripts/orbits/Orbit.cs

@@ -0,0 +1,12 @@
+using Godot;
+using System;
+
+public class Orbit
+{
+    public Orbit(PointMass a, PointMass b)
+    {
+
+    }
+
+    // ellipse
+}

scripts/orbits/OrbitSystem.cs

@@ -0,0 +1,109 @@
+using Godot;
+using System;
+
+public class OrbitSystem : Node, IMassive, ILocation
+{
+    [Export] private NodePath _a;
+    [Export] private NodePath _b;
+    [Export] private NodePath _barycenter;
+
+    private readonly PointMass[] _pointMasses = new PointMass[2];
+
+    public Vector3 Position => Barycenter;
+
+    [Export]
+    public Vector3 Barycenter
+    {
+        get
+        {
+            var p0 = _pointMasses[0].Position;
+            var p1 = _pointMasses[1].Position;
+
+            return p0.LinearInterpolate(p1, .5f);
+        }
+        set => _ = value;
+    }
+
+    public float Mass { get; } = 1;
+
+    private Orbit _orbit;
+
+    private ImmediateGeometry _orbitGeometry;
+
+    public override void _Ready()
+    {
+        InitPointMasses();
+        InitGeometry();
+    }
+
+    public override void _Process(float delta)
+    {
+        DrawOrbit();
+    }
+
+    private void InitOrbit()
+    {
+
+    }
+
+    private void DrawOrbit()
+    {
+        int steps = 100;
+
+        _orbitGeometry.Clear();
+        _orbitGeometry.Begin(Mesh.PrimitiveType.LineLoop);
+        _orbitGeometry.SetColor(new Color(1, 0, 0));
+
+        var ellipse = new Ellipse(1, .5);
+
+        for (int i = 0; i < steps; i++)
+        {
+            float t = i / (float)steps;
+            float a = t * Mathf.Tau;
+
+            var v2 = ellipse.Focus0 + ellipse.GetPosition(a);
+
+            _orbitGeometry.AddVertex(new Vector3
+            {
+                x = (float)v2.X,
+                z = (float)v2.Y
+            });
+        }
+        _orbitGeometry.End();
+    }
+
+    private void InitGeometry()
+    {
+        _orbitGeometry = new ImmediateGeometry();
+        var m = new SpatialMaterial();
+        m.VertexColorUseAsAlbedo = true;
+        m.FlagsUnshaded = true;
+        _orbitGeometry.MaterialOverride = m;
+        AddChild(_orbitGeometry);
+    }
+
+    private void InitPointMasses()
+    {
+        var a = GetPointMass(_a);
+        var b = GetPointMass(_b);
+
+        if (a.Mass > b.Mass)
+        {
+            _pointMasses[0] = a;
+            _pointMasses[1] = b;
+        }
+        else
+        {
+            _pointMasses[0] = b;
+            _pointMasses[1] = a;
+        }
+    }
+
+    private PointMass GetPointMass(NodePath path)
+    {
+        var spatial = GetNode<Spatial>(path);
+        //var massive = node.GetChild<IMassive>(0);
+
+        return new PointMass(spatial, 1f);
+    }
+}

scripts/orbits/Planet.cs

@@ -0,0 +1,18 @@
+using Godot;
+using System;
+
+public class Planet : Node, IMassive
+{
+    [Export]
+    public float Mass { get; set; }
+
+    public override void _Ready()
+    {
+
+    }
+
+    //  public override void _Process(float delta)
+    //  {
+    //
+    //  }
+}

scripts/orbits/PointMass.cs

@@ -0,0 +1,25 @@
+using Godot;
+
+public struct PointMass : IMassive, ILocation
+{
+    public float Mass => _massive == null ? _mass : _massive.Mass;
+    public Vector3 Position => _spatial.Translation;
+
+    private IMassive _massive;
+    private Spatial _spatial;
+    private float _mass;
+
+    public PointMass(Spatial spatial, IMassive massive)
+    {
+        _spatial = spatial;
+        _massive = massive;
+        _mass = 0;
+    }
+
+    public PointMass(Spatial spatial, float mass)
+    {
+        _spatial = spatial;
+        _massive = null;
+        _mass = mass;
+    }
+}

scripts/orbits/math/Ellipse.cs

@@ -0,0 +1,63 @@
+using Godot;
+using System;
+using Vim.Math3d;
+
+public struct Ellipse
+{
+    public Ellipse(double a = 1, double e = 0)
+    {
+        SemiMajorAxis = a;
+        Eccentricity = e;
+
+        // TODO: this is an immutable struct, so initialise everything else 
+        // in the constructor to avoid recalculating properties whenever
+        // they are accessed
+    }
+
+    /// <summary>
+    /// Semi-major axis
+    /// </summary>
+    public double a => SemiMajorAxis;
+    public double SemiMajorAxis { get; }
+
+    /// <summary>
+    /// Eccentricity
+    /// </summary>
+    public double e => Eccentricity;
+    public double Eccentricity { get; }
+
+    /// <summary>
+    /// Semi-minor axis
+    /// </summary>
+    public double b => SemiMinorAxis;
+    public double SemiMinorAxis => Math.Sqrt(Apisides.max * Apisides.min);
+
+    /// <summary>
+    /// Get a position on the auxiliary circle
+    /// </summary>
+    /// <param name="t">Angle in radians around the circle</param>
+    /// <returns>2D position on the circle scaled by the semi-major axis</returns>
+    public DVector2 GetAuxiliaryPosition2D(double t = 0) =>
+        new DVector2(Math.Cos(t), Math.Sin(t)) * a;
+
+    private Apisides Apisides => new Apisides(a, e);
+
+    public DVector2 GetPosition(double t) =>
+        new DVector2(Math.Cos(t) * a, Math.Sin(t) * b);
+
+    public DVector2 Focus0 => new DVector2(-GetFocusDistance(), 0);
+    public DVector2 Focus1 => new DVector2(GetFocusDistance(), 0);
+    private double GetFocusDistance() => Math.Sqrt(a * a - b * b);
+}
+
+public struct Apisides
+{
+    public readonly double min;
+    public readonly double max;
+
+    public Apisides(double a, double e)
+    {
+        min = a * (1 - e);
+        max = a * (1 + e);
+    }
+}

scripts/trains/Railway.cs

@@ -0,0 +1,119 @@
+using Godot;
+using System;
+
+[Tool]
+public class Railway : Node
+{
+    [Export]
+    private NodePath[] _railPaths;
+
+
+    [Export]
+    private NodePath _centreLine;
+
+
+    private float _railWidth;
+    private float _railHeight;
+    private float _railGauge;
+
+    [Export]
+    private float RailWidth
+    {
+        get => _railWidth;
+        set
+        {
+            _railWidth = value;
+            if (Engine.EditorHint)
+            {
+                SetRailCrossSection();
+            }
+        }
+    }
+    [Export]
+    private float RailHeight
+    {
+        get => _railHeight;
+        set
+        {
+            _railHeight = value;
+            if (Engine.EditorHint)
+            {
+                SetRailCrossSection();
+            }
+        }
+    }
+
+    [Export]
+    private float RailGauge
+    {
+        get => _railGauge;
+        set
+        {
+            _railGauge = value;
+            if (Engine.EditorHint)
+            {
+                SetRailCrossSection();
+            }
+        }
+    }
+
+    private Path _path = null;
+    private Path Path
+    {
+        get
+        {
+            if (_path == null)
+            {
+                _path = GetNode<Path>(_centreLine);
+            }
+            return _path;
+        }
+    }
+
+    public Curve3D Curve => Path.Curve;
+
+    // the centre line should not have geometry directly associated with it.
+    // instead, two additional path/csgpolygon combinations should be managed
+    // in reference to the centre line for the left and right rails.
+
+    // Called when the node enters the scene tree for the first time.
+    public override void _Ready()
+    {
+
+    }
+
+    private void SetRailCrossSection()
+    {
+        // error checkin
+        if (_railPaths.Length != 2)
+            throw new Exception("need 2 rails");
+
+        // set rail geometry
+        for (int i = 0; i < 2; i++)
+        {
+            var rail = GetNode<CSGPolygon>(_railPaths[i]);
+
+            float w = RailWidth;
+            float h = RailHeight;
+            // horizontal offset of rail from centreline
+            float c = (-.5f + i) * RailGauge;
+
+            var polygon = new Vector2[4];
+            polygon[0] = new Vector2(c - w, 0);
+            polygon[1] = new Vector2(c - w, h);
+            polygon[2] = new Vector2(c + w, h);
+            polygon[3] = new Vector2(c + w, 0);
+            rail.Polygon = polygon;
+        }
+    }
+
+    public override void _Process(float delta)
+    {
+
+    }
+
+    public void AddPathFollower(PathFollow pathFollow)
+    {
+        Path.AddChild(pathFollow);
+    }
+}

scripts/trains/Train.cs

@@ -0,0 +1,121 @@
+using Godot;
+using System;
+using System.Collections.Generic;
+
+public class Train : Spatial
+{
+    [Export]
+    private float _speed = 1.0f;
+
+    [Export]
+    private NodePath _railwayPath;
+
+    private Railway _railway;
+    private float _distance = 0;
+
+    [Export]
+    private NodePath _carPath;
+    private TrainCar _car;
+
+    private Spatial[] _bogies;
+    private readonly Dictionary<Spatial, PathFollow> _bogiePathFollows = new Dictionary<Spatial, PathFollow>();
+
+    private ImmediateGeometry _imgui;
+
+    // Called when the node enters the scene tree for the first time.
+    public override void _Ready()
+    {
+        _imgui = new ImmediateGeometry();
+        var m = new SpatialMaterial();
+        m.VertexColorUseAsAlbedo = true;
+        m.FlagsUnshaded = true;
+        _imgui.MaterialOverride = m;
+        AddChild(_imgui);
+
+        _railway = GetNode<Railway>(_railwayPath);
+        _car = GetNode<TrainCar>(_carPath);
+
+        _bogies = new Spatial[_car.Bogies.Length];
+        for (int i = 0; i < _bogies.Length; i++)
+        {
+            var b = _car.Bogies[i];
+            _bogies[i] = b;
+            _bogiePathFollows[b] = new PathFollow
+            {
+                RotationMode = PathFollow.RotationModeEnum.Oriented
+            };
+            _railway.AddPathFollower(_bogiePathFollows[b]);
+
+            // detach the bogies from the car as we need to set the bogie
+            // positions in order to calculate the car position and it will
+            // get confusing if they're in a parent/child relationship,
+            // so make them all siblings
+            _car.RemoveChild(b);
+            this.AddChild(b);
+        }
+    }
+
+    // Called every frame. 'delta' is the elapsed time since the previous frame.
+    public override void _Process(float delta)
+    {
+        var length = _railway.Curve.GetBakedLength();
+
+        // first determine which is the front bogie. this is dependent
+        // on the direction of travel.
+        int travelDir = (int)(_distance / length) == 0 ? -1 : 1;
+
+        DrawTravelDirectionIndicator(travelDir);
+
+        // keep track of total distance
+        _distance += delta * _speed;
+        // wrap at 2x total distance
+        _distance %= length * 2;
+
+        var distance = PingPong(_distance, length);
+
+        // get the position of the first bogie on the track
+        var b = _bogies[0];
+        var pathFollow = _bogiePathFollows[b];
+
+        pathFollow.Offset = distance;
+        b.Translation = pathFollow.Translation;
+        b.Rotation = pathFollow.Rotation;
+
+        // now we want to position the car over the first bogie
+        GD.Print(_car.Wheelbase);
+        _car.Translation = b.Translation;
+        _car.Rotation = b.Rotation;
+
+        // TODO: this rotation quat needs to position the other 
+        // bogie over the curve
+        _car.Translation += _car.Transform.basis
+            .RotationQuat()
+            .Xform(Vector3.Back) * _car.Wheelbase * 0.5f;
+    }
+
+    private void DrawTravelDirectionIndicator(int travelDir)
+    {
+        _imgui.Clear();
+        _imgui.Begin(Mesh.PrimitiveType.LineStrip);
+        _imgui.SetColor(new Color(1, 0, 0));
+        var c = _car.Translation + Vector3.Up * 2;
+        _imgui.AddVertex(c);
+        var d = travelDir;
+        var l = 5;
+        var e = c + _car.Transform.basis
+            .RotationQuat()
+            .Xform(Vector3.Forward) * d * l;
+        _imgui.AddVertex(e);
+        _imgui.End();
+    }
+
+    // poor mans branching ping pong
+    private float PingPong(float t, float length)
+    {
+        var tml = t % length;
+
+        return t > length
+            ? length - tml
+            : tml;
+    }
+}

scripts/trains/TrainCar.cs

@@ -0,0 +1,90 @@
+using Godot;
+using System;
+
+[Tool]
+public class TrainCar : Spatial
+{
+    public float Length => Box.Depth;
+
+    [Export]
+    private NodePath _boxNode;
+
+    [Export]
+    private NodePath[] _bogiePaths;
+
+    [Export]
+    public float Wheelbase
+    {
+        get => _wheelbase;
+        set
+        {
+            _wheelbase = value;
+
+            // don't set the wheelbase except from the editor
+            if (!Engine.EditorHint) return;
+
+            SetBogeyPositions();
+        }
+    }
+    private float _wheelbase;
+
+    public Spatial[] Bogies
+    {
+        get
+        {
+            if (_bogies == null)
+            {
+                _bogies = new Spatial[2];
+                for (int i = 0; i < _bogiePaths.Length; i++)
+                {
+                    _bogies[i] = GetNode<Spatial>(_bogiePaths[i]);
+                }
+            }
+            return _bogies;
+        }
+    }
+    private Spatial[] _bogies = null;
+
+    private CSGBox _box;
+    private CSGBox Box
+    {
+        get
+        {
+            if (_box == null)
+            {
+                _box = GetNode<CSGBox>(_boxNode);
+            }
+            return _box;
+        }
+    }
+
+    public Railway Railway { get; set; }
+
+    public override void _Ready()
+    {
+
+    }
+
+    public override void _Process(float delta)
+    {
+    }
+
+    private void SetBogeyPositions()
+    {
+
+        if (Wheelbase > Length)
+            throw new Exception("wheelbase cannot be longer than car length");
+
+        if (_bogiePaths.Length != 2)
+            throw new Exception("_bogeyPaths should be of length 2");
+
+        for (int i = 0; i < 2; i++)
+        {
+            _bogies[i] = GetNode<Spatial>(_bogiePaths[i]);
+        }
+
+        var axis = Vector3.Forward * Wheelbase;
+        _bogies[0].Translation = axis * 0.5f;
+        _bogies[1].Translation = axis * -0.5f;
+    }
+}