Why did virtually all the railroads in the northern U.S. use the same rail gauge BEFORE regulation of the railroads? Ah -- you specify the ``northern'' U.S. The situation in the south was very different. And even in the north, the Pennsylvania Railroad was so large (they're the ones who billed themselves as ``the standard railroad of the world) that other folks had to follow if they came near the PRR. It was near-monopoly that created that situation, not any desire for co-operation. In Europe, there are still a variety of different gauges, electrical standards, loading gauges, etc. (Actually, the latter two are problems in the U.S. as well.)