I have just read the excellent “Tracks Across Continents – Paths Through History” by Douglas J Puffert (2009 University of Chicago Press). This book explains why the rail gauge of 4 feet 8.5 inches has become dominant for mainline railways throughout the world. Track gauge is the distance between the inside faces of the two rails of a railway and, obviously, the rails have to be the same distance as the wheels of the train that runs on it. Traffic between any two point on a rail network will clearly be inconvenienced if different track gauges are needed for part of the journey as the cargo must be moved between two trains or somebody needs to perform a cunning “hack”to deal with this break of gauge.
Engineers have differing opinions as to what the ideal track gauge would have been, but many think a little more than the Stephenson Gauge (as Puffert calls it) would have been better.
Puffert, Tracks across Continents – Paths through History