This is not original with me, and might be on the web somewhere, but I like it a lot: Two trains travel nose to nose at the same speed along adjacent parallel gradeless tracks. The trains are identical in construction, maintainance, cargo distribution, fuel load, etc. When they approach the depot, the engineer on on one train applies the brakes gradually and comes to a slow stop. A while later, the engineer on the other train applies the brakes more abruptly and comes to a rapid stop, but nose to nose with the other train. Both trains take on cargo, and offload other cargo, and are refueled. They still have identical configurations. The engineers start up their engines. The train that stopped abruptly leaves, but the train that came to a gradual stop is unable. The engineer on that train then backs up the train for a while, stops, and is then able to go. What gives?