Let a former Pittsburger take a crack at this. Your right that the demise of the old steel industry more or less did away with the ng mill dinks. The old complexes covered hundreds of acres and were connected with both ng and standard gauge lines. The newer plants are a continious operation. Raw materials (or scrap) in one end and the finnished product out the other. No need for a extensive intraplant railroad.
Porter had a complete line of mill locos both in steam and infernal combustion, as well as the other builders.