I was mostly referring to the differences in handling the longer trains we ran with the box cars compared to the short trains we normally run now with the newer coaches.
You are correct in thinking the new cars are heaver, and that concentrates the tonnage in a shorter train. Fewer cars also means less slack in the train. The short, heavier trains will roll through curves more easily because there are fewer wheels to bind in the curve. The longer trains are more often wrapped around two or three curves and through sags or humps in the grade all at the same time. This can cause the slack to be bunched in part of the train and stretched in another. The engineer has to keep this in mind and plan ahead as to how to handle the train to control the slack and give a smooth ride. It often requires braking
and using power to keep the train stretched where a shorter train may be allowed to drift through the same area. This is not to say that the shorter trains don't require skilled handling, there are often just fewer variables to consider in doing it.
On the other hand, a long train can help even out the grade changes and be easier to handle in certain areas. A short train will be more immediately affected by grade changes and require more effort to maintain a steady speed.
The key to smooth train handling in all cases is for the engineer to know the road in detail and understand how a given train will behave in each location.