[electricron]

The main reason to standardize the gauge is to reduce the number of car transfers required with huge numbers of freight cars, thereby reducing the costs of shipping on a national scale. Shippers on the BNSF line in Fort Worth should be able to ship to customers on the UP line in San Antonio, without having to pay to change cars or change a car's gauge.

Stand alone transit systems in a city could operate on any gauge, as long as that transit vehicle never leaves any individual line or group of lines using the same gauge. Boston has commuter (MBTA), subways (red & orange lines), and light rail (green lines) that do not change the lines they run on. The red & orange subway lines are standard gauge, but the green line is narrow gauge. There is no operational reason to rebuild the green line to standard gauge.