I loved those old steam trains!
I remember riding on steam engine passenger trains in the forties. When the engine first started pulling, there was still slack in all the couplings. You could hear the snaps coming from cars nearer the engine as, one by one, their couplers pulled taut. Then, bang, you got yours, and began a slow roll. From some reason, diesels just start rolling.
Steam locomotives have pistons pushing a crankshaft that turns the wheels. They develop maximum torque in the middle of their stroke, and virtually none at each end of the stroke. If they try to pull a loaded train with the couplings tight, the wheels just slip when the crankshafts reach peak torque. This wears out the tires and the track. A diesel-electric locomotive puts out a nice even torque, and the engineer can just increase the torque until the train starts to move without slipping the tires.
Tires??? Did he say tires? Yes. I put that in to prove I know what I'm talking about. Look up "railway tire" on the internet. It's the steel rim on the steel wheel of a rail car.
I come from a long line of railroaders and have ridden in steam engines cabs all over the world. Engineers (electric, diesel-electric or otherwise) are taught to “take out the slack” whenever possible as they start trains. There are exceptions, but doing so makes things easier on passengers and cargo, and most of all, prevents yanking a coupler out of its pocket. Very embarrassing! Leaves part of the train standing and air brakes set on both pieces.
I reaslly like your description - brings back memories of train yards in Chicago and Denver.