Another issue is buffer. When you have an automatic coupler like in North America, that also serves as buffer. But European trains normally have two buffers on both sides of their coupler. Many of the new Scharfenberg coupler trainsets lack side buffers, which calls for special safety considerations.

If we are here, trains can differ in another thing: the longitudinal forces they can withstand. The US, with its much longer trains, demands much higher values than is the standard in Europe. But Europe also has railbuses and some trainsets which don't fit that norm, and for that reason can't be put in a long train or 'bumped' by a shunter.

_{*Lunatic*, n.
One whose delusions are out of fashion.}