shmodot/scripts/Train.cs

using Godot;
using System;

public class Train : Spatial
{
    [Export]
    private float _speed = 1.0f;

    [Export]
    private NodePath _railwayPath;

    private Railway _railway;
    private readonly PathFollow _pathFollow = new PathFollow
    {
        RotationMode = PathFollow.RotationModeEnum.Oriented
    };
    private float _distance = 0;

    // Called when the node enters the scene tree for the first time.
    public override void _Ready()
    {
        _railway = GetNode<Railway>(_railwayPath) as Railway;
        _railway.AddPathFollower(_pathFollow);
    }

    // Called every frame. 'delta' is the elapsed time since the previous frame.
    public override void _Process(float delta)
    {
        var length = _railway.Curve.GetBakedLength();

        // keep track of total distance
        _distance += delta * _speed;
        // wrap at 2x total distance
        _distance %= length * 2;

        var distance = PingPong(_distance, length);

        _pathFollow.Offset = distance;
        Translation = _pathFollow.Translation;
        Rotation = _pathFollow.Rotation;
    }

    // poor mans branching ping pong
    private float PingPong(float t, float length)
    {
        var tml = t % length;

        return t > length
            ? length - tml
            : tml;
    }
}