Another factor in the end of steam was spare parts.
As railroads with fleets of low-tech, low efficiency locomotives dieselized early, the PRR being a prime example, less manufacturers were producing parts for the surviving steam. The NKP Berks successfully defeated the EMD FTs in a head to head trial, but their maintenance costs went up as the supply of parts decreased.
The Erie railroad was a good example of a broad gauge railroad...as well as a railroad that never should have been built. The Erie started off as a 5' gauge line, and maintained all of their original clearances as they could haul oversize freight easier than other roads. Unfortunately, Lima, Ohio, pop 50,000, was about the largest town aside from the major destinations of NYC, Chicago, and Cleveland.
Trivia: Does anyone know why standard gauge is 56.5" gauge and not 57" or 56"?
Answer: It was based on the Roman Chariot width.