The US standard railroad gauge is 4 feet, 8.5 inches.
Why?
Because the first railroads were built in Europe.
On roads constructed by the Romans.
And the Romans constructed the roads to be wide enough for chariots going opposite directions to pass each other on the same road, bridge, tunnel, or underpass.
The width between the wheels of Roman wagons was 4 feet, 8.5 inches.
The Roman roads had been in use in Europe since before Jesus. So, when the trains came along, the Europeans had a choice: build trains with a gauge of 4 feet, 8.5 inches or widen every road, bridge, underpass, and tunnel in Europe.
Then, when America constructed railroads, we used British locomotives that had a gauge of 4 feet, 8.5 inches.
So, we constructed our railroads bridges, tunnels, and underpasses to accommodate the trains with the gauge of 4 feet, 8.5 inches.
Next time you see space craft blast off from Cape Canaveral, keep in mind that those rocket engines had to come by train down tracks that were 4 feet, 8.5 inches apart.
So, to this day, the Romans are still with us, every 4 feet, 8.5 inches of the way!
For a railroad adventure!!
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(I autograph all my books from this site.)