The S2 was a yard switcher built by Alco (American Locomotive Company). The Pennsylvania Railroad acquired the S2 in 1950. Their engines were rated at 1000HP. The PRR class was AS10 (Alco, switcher, 1000 horsepower). The photo of road number 5642 came from the Fallen Flags web site. During WW-II Alco was designated the main company for building yard switchers, so most railroads wound up with either an S1 or an S2. They were very reliable and many survive even still today (source: Model Railroader Magazine, March 1984, pg. 33).

The Arnold model was released in 1991, with "Rapido" couplers and wheel flanges that were way too tall for any track, really. With the new motor installed it runs quiet and slow, and with the flanges turned-down, it actually runs on code 40 track. The model weighs 2.3 oz (66g). The model photos below show all of the modifications I have made to my model.

Print References:
"Locomotives: Improving S-2 Operations", by Ron Bearden (N-Scale Magazine 1991, 07/08).
"Lights for the Arnold S-2", by Max Magliaro (Railroad Model Craftsman 1992, 02).
"S-2 Gearing Improvement", by Ron Bearden (N-Scale Magazine 2002, 11/12)