The UP sets with the two regs attached externally to either side of the mainstock remind me somewhat of the the SRBs on the US Space Shuttle. These are the large rockets mounted on either side of the main fuel tank. They are filled with solid rocket propellant, thus Solid Rocket Boosters - SRBs.
The SRBs are made by a company called Thiokol in Utah. The Thiokol engineers wanted to build the SRBs wider but were constrained. The SRBs are transported from Utah to the launch site via train, and part of the track goes through a railway tunnel. This tunnel was made to just barely accommodate a train running on standard gauge US railway tracks. The standard width of US railway tracks is 4’ 8.5".
Now, isn’t that an odd size? You may be wondering why the US settled on this size of track as their standard railway gauge. It turns out that the US railways were built by expatriate Englishmen, who simply stayed true to the tramway gauge they had learned and used in England.
So why would England have set such an odd size as a standard gauge for their tramways? Well, it turns out that this was the standard gauge for making horse drawn wagons. The companies hired to build the tram cars simply re-used all their existing jigs and tools from building horse drawn wagons for building tram cars.
But why, you might ask, did the English build their wagons to this gauge? Because that was the gauge of the ruts in the English highway system of the time. If you didn’t want your wagon wheels to be smashed to pieces, you made your wagons to suit the size of the ruts.
And what would cause the ruts in England’s highways to be of this odd size? Well, many of Englands main highways were actually built over the top of the original roads layed down by the Roman legions centuries ago, and the ruts in the Roman roads were also of this odd dimension.
But what would cause ruts in the Roman roads, you ask? Why, chariots, of course! The Roman standard for the size of their chariots was exactly 4’ 8.5", or in other words, exactly the size of the backsides of two horses pulling side by side.
What does all this tell us? Bureacracies live forever!
(Don’t laugh. It may even be true!)
djm