A NASA spokesperson confirms that railroad tunnel dimensions were a constraint that had to be taken into account when designing the SRBs. However, tunnel dimensions are less a function of track gauge than of rolling stock width. U.S. railroad cars are quite a bit wider than those in England because parallel tracks are placed farther apart. (I’m talking tracks, not rails here, capisce?) As a consequence, U.S. railroad tunnels typically are wider too. So you can’t really make the case that the size of the space shuttle’s boosters was determined by the width of a couple horses’ butts.
Not quite how it works. The determining factor is the loading, or structure gauge - the maximum height and width of rollingstock. It is these dimensions that are typically much larger on US railroads, the distance between track centres on double track is usually about 13 feet in both countries. Even so, all railways have provision for the handling of out-of -gauge loads, so oversize lading such as shuttle boosters may be moved by rail.