These are called solid rocket boosters, or SRBs. Well, Imperial Rome built the earliest roads in Europe and Great Britain, parts of which are still in use today. Some of the roads were intentionally grooved with ruts in difficult parts to guide Roman vehicles. Time passed, and in the 16th century, tramways began to be developed in the British Isles to haul coal, ore, or stone. Many railroads in North America were designed by English expatriates, who used this gauge and many others.
Trains go through tunnels too, even more than highways do.
The SRBs used on the Space Shuttle are manufactured by a company called Thiokol Propulsion Group in Utah. Utah is on the other side of those big honking mountains from Florida, where the space shuttle is launched, and SRBs have to be shipped by train, in sections. It's possible that the engineers would have liked to design the booster rockets to different dimensions, but they can't, because the standard was first set over 2,000 years ago, by the width of a horse's ass. So every time you encounter a new standard, you can expect that it may have been set by a horse's ass, or a committee of them. There were 400 hits, and each one was nearly identical to the others.