MPW,

This is how they did regauage at one time at the Spanish-French border, but only for a limited set of 1st class train equipment. Engines were changed out. Most trains, you simply changed trains at the Spanish border. All of the new Spanish HST lines are built to Standard guage, so they just go on through to Barcelona, Madrid, and Sevilla, and soon Alicante.

It is my understanding that the US Army engines, while designed to be variable in guage, should the need arise, had to do this in a "shop" setting, they didn't just push some button and it "magically" happened. This includes the broad guage Alco Soviet lend lease engines, many of which travelled via the Iranian railroad to reach the Soviet Union, where they were regauged.

This is also why almost every nation building new or doing significant rebuild (except Russia and India where the broad guage systems are huge, or Southern Africa where the 3'6" systems are also huge) are building or relaying to standard guage (Spain, Australia, Argentina, Brazil).

It also gets to the oft heard criticism of BART because, there is more competitive "off the shelf" transit and rail equipment available in standard guage, allowing more competition/less of a premium for customization, in the marketplace.