Finding relative position across connected splines

[DekoGames]

Is there a simple way to determine the relative position of two spline controllers on separate, connected splines?   I'd like to determine what position a train car is within a train (first, second, third, etc.) when the train itself may span multiple splines.

I'm guessing that I'll need to create a list of the track splines (or a lookup table) and assign values to each spline to define 'west' and 'east' relative positions for each, but I'm wondering if there is an easier solution.