ZenBeam, someone reminded me about rolling friction, so I see what you mean now, and glad you found your answer.
Some old guy may have said it was inertia, but that is what this thread is discussing: is it really inertia that’s the culprit?
Consider for a moment if the train cars used skis instead of wheels on rails, and slid along a frictionless icy surface. That does not change the inertia, the cars resistance to change in motion. It does, however, remove the resistance of friction. In that case, the engine can apply force to move the cars, and there is no force opposing that motion. Inertia just means that the acceleration rate of the entire train will be very slow, because the force of the engine is limited by the friction of the drive wheels to the rails.
Momentum does come into play in the standard train scenario, because momentum is used to overcome the friction of the system. As I described above, the successive chain of impacts allows the momentum to provide the hump to overcome the friction of the system.
I suppose one can be sloppy with the word “inertia” and include friction as “resistance to change in motion”, but that is not what is meant by Newton in his First Law.
Yes, because you have the cummulative kinetic energy of the engine and all the preceeding cars to create stress in the couplers at the end of the train.
Thus, the need to control speed to keep the stresses low.