It's possible to have different gauges at the same station, requiring just a change of train rather than an additional bus transfer. And some systems are variable gauge, allowing for the same train to operate on different systems.

https://en.m.wikipedia.org/wiki/Variable_gauge

Back in the day choosing your rail gauge had a large input from the military, since it was strategically disadvantageous to have the enemy supply lines easily extended into your heartland. This was before the days of air forces or even proper highway systems, so logistics depended largely on horse drawn wagon trains.

It's not so different from the Soviets choosing an ever so slightly different caliber for their mortars so it's not compatible with the ones from NATO.

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

> 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.