> But wouldn't you want your trains to easily extend into theirs?

Can’t find the link now but I read a fascinating article about the German and Russian use of railways on the WW2 Eastern Front. Long story short IIRC: the Russian rail network was primitive by German standards, primarily lacking the automated signalling that allowed relatively high speed trains. Primitive, but it still worked and was very resilient to damage. The Russians destroyed a lot of their railways as they retreated and could quickly rebuild it when their fortunes changed.

Also, the Russians operated their trains using a centralised planning function that could prioritise strategically important routes and cargoes. In contrast, senior German commanders could in effect commandeer trains to support their local tactical needs, sometimes playing havoc with the overall network.

Most countries plan their military around defense of their territory though, planning around military aggression of their neighbours has only happened for short periods and in a few countries since the invention of railways.