It's interesting to study about how different nations considered the military applications of railroads.

Germany, as explained, probably was the most offensive minded when it came to using their trains. A country like Russia, on the other hand, may have thought defensively when it built its rail lines to a different gauge so as to preclude an invader (read Germany) from using its own rolling stock on those lines.

Great Britain, while not really having to worry too much about a land invasion, did have to consider what their more restrictive loading gauge meant for the size of military equipment, for example, not building trucks or combat vehicles that would exceed the height of bridges or the dimensions of tunnels.