SneerClub

Definitely this. There are few more persistent myths out there than the origin of various track gauges -- starting with the myth that standard gauge (1435mm) derives from the width of a horse's arse. Military planners weren't stupid enough to think that a break of gauge would present an insurmountable obstacle to an invading army (not least because they could just commandeer or seize rolling stock and march all their soldiers off one train and onto another), but instead relied on the obvious, reliable stuff like having plans to blow up bridges and tunnels to deny the enemy the use of the infrastructure. If a gauge is unusual like in Russia or Ireland the most likely cause is that it was a compromise between people who wanted broad gauge and people who wanted Stephenson gauge which resulted in the choice of some number inbetween the two. Russian gauge is a round 5', Irish gauge is 5'3", both round numbers in archaic units.

There are loads of weird things which essentially boil down to "someone made an arbitrary decision" - for instance, a lot of railways in Western European countries have left-hand running for no reason other than it was what George Stephenson used when he built the Liverpool & Manchester Railway and British practice was highly influential on the early development of railways on the continent.