All you need to do to calculate the position for each axle is to use a percent value 0 to 1 to control the distance along the spline from point to point.
Edit actually this won’t work entirely if the spline points are further or shorter apart, only if the distances are equal. Hmm, interesting problem.
Edit2 : Just placing the axles at the correct distance apart and moving them at the same speed will keep them at their correct position. If the carriage is attached to both the axles by a physics ball joint component each in 1 axis you will get realistic axle rotation under the carriage. could limit the rotation in a second axis to get the up and down rotation from going up and down.
And to apply physics, toggle it on and off, I don’t think even the simulation train games are physically modeling a axle wheel / rail system to propel itself. Maybe but i highly doubt it.