I've been working for more than a year on reproducing the Pennsylvania Railroad in Railway 32. For those who don't know, the PRR was a pre-eminent railroad company, serving a large portion of the north-eastern United States. It vanished in 1968 in a merger with its arch-rival, the New York Central; the merger was a disaster, and the merged company, Penn Central, declared bankruptcy within a few years. From the ashes, Conrail was created. In recent years, Conrail was jointly bought by the two largest railroads on the east coast, Norfolk Southern and CSX, and split in half.
The PRR has always been a fascinating subject for me, since it could be argued to have had the most 'personality' of all American railroads. Whether one agrees with this or not, it's certainly the case that the PRR (the 'Pennsy') had its own way of doing things. The Pennsy rarely bought off-the-shelf locomotives from the major locomotive builders. Many of its locomotives were designed and built in-house; even those bought from other builders, mostly Baldwin, were very much done in the PRR 'house style'.
PRR steam locomotives were very standardized. PRR advertising called the company the "standard railroad of the world" -- meaning both that the PRR was the standard other railroads tried (and failed) to meet, and also that standardization and uniformity was the secret of the PRR's success. While other roads bought their locomotives in small batches, each batch different from the last, the PRR produced huge fleets of locomotives, all largely alike.