The US standard railroad gauge (distance between the rails) is 4 feet 8.5 inches. Yes, it is an odd dimension and comes from British history. You see, the English expatriates built the US Railroads and those were the gauge dimensions used in England. The reason why the English used that gauge is because the first railway tracks were built by the same people who built the pre-railroad tramways, and that’s the gauge they used.

To built the tramways, the English used the same jigs and tools and wheel spacing, that were used for building wagons. They couldn’t use any other spacing, because the wagon wheels would break in the spacing of the wheel ruts which were common on old roads around England.

Note:

Model Railroads is a US term
Model Railways is a British term

For information on HO Model Trains (OO in Britain)