OT- Rail specifications and their effects

[Silent Hunter UK]

Originally Posted by Sanjuro

The UK also has had differing rail gauges; various narrow gauge railways often for mountain use, or within factories (indeed, even the National Shooting Centre at Bisley used to have its own railway to connect to the mainline station at Brookwood) and the Great Western Railway was originally built with 9' gauge.
4'8.5" won though: a better compromise of stability with the ability to turn corners with a reasonable radius. If logic had been applied, perhaps 5' would have been settled on. Rather than describe tunnels as "slightly wider than the gauge" it is probably more accurate to say tunnel width is an indirect function of track gauge.

Tunnel sizes can differ even on standard gauge - look up "loading gauge" for more info.