The original post: /r/askscience by /u/MrDirtNP on 2024-11-13 20:52:52.

I have three trains (X, Y and Z) of equal proportions on separate parallel tracks in space. Each train is equipped with measurement tools to keep track of the speed, length and direction of the other trains.

Train X stands still while Train Y goes with 50% light speed in one direction while Train Z goes with 50% light speed in the opposite direction. How fast is Train Y relative to Train Z? What would happen when we add even more speed to each train? (Train X is just an anchor point)

Common sense would say 0.5c+0.5c=1.0c but then 0.6c+0.6c=1.2c and that’s impossible, is it?