I have no idea on this one. Since it is Saturday afternoon, I’ll hear the answer tomorrow on the show probably. Still time to speculate though. My guess is that since I’ve never see a train without a caboose at the end – sometimes I’ve seen an engine go down the track with only a caboose, and no other cars, but never just an engine by itself – that there is something the caboose does that is important to the function of the engine. And with no caboose, whatever that is, it isn’t happening.
There is a difference between static friction and dynamic friction. The force of friction due to a rolling train car is lower than the force required to break the static friction force. After a stop, all the slack in the couplings is compressed, when the engine now starts pulling the train again only one car at a time exterts static friction. The rest ahead of the fully stopped car are exerting only dynamic friction.
When the engineer halted the pull because the caboose was dragging an axle, all the slack was extended out of the couplings. The engine did not have enough tractive effort (pull force) to break the combined static friction of all the cars. What he/she needs to do is reverse; recompress the couplings, rereverse forward to do the progressive coupling extension to get the train under way.
You are spot-on @Researcher. That was another good puzzler offered up by the Car Talk guys, as – at least in retrospect – it is something I could have thought of had I been more clever. I was definitely off-base with my “it’s got to be something about the caboose” guess!
I don’t know about other train companies but CSX railroad retired their cabooses many years ago. A lot of people around here bought some of the old ones and use them for a variety of reasons like lawn art, advertising, playhouses and storage. The caboose has no function at all as far as the function of the engine. They were used to transport the conductor (who is in charge of the train by the way…not the engineer) and switchman. In years past they also transported the brakeman. I haven’t seen a caboose on the rails in many years.