The Alco S4 was distinguished from the S2 by the trucks, and from the S3 by the large radiator intake grills. The S2 rode on Blunt trucks, and the S4 had AAR Type A trucks. The S3 ( and S1 ) had a shorter radiator ( taller than it was wide ), and a straight pipe-like exhaust stack, indicating that the S1 and S3 were not turbocharged. It had a model 539 six cylinder McIntosh & Seymour diesel engine. The 5 in the diesel model number indicates 12½ X 13 inch cylinders and the 39 denotes the first year the diesel was fired up on the test stand.

The Bramberger was an electric railroad running between Ogden and Salt Lake City, Utah, but required a diesel to move troop trains in and out of Hill Air Force Base, which had no overhead wire. In service on the Bramberger, the 570 was equipped with a trolley pole at each end to activate the Nachod block signals. The War Production Board allocated the RS1 because the Bramberger was built to interurban standards which would not permit a long wheel base steam locomotive. After the war, the RS1, which had seen considerable service, was in need of an overhaul, first performed by Bramberger's shop forces, which lasted about four years before it was in need of a more extensive rebuilding. Julian Bramberger (president) approached EMD about rebuilding the locomotive (he happened to own stock in GM), and EMD agreed to rebuild the RS1, its third rebuilding effort. The 570 returned from EMD in December 1951 with a 1200 horsepwer 567B derated to 1000 horsepower and, of course, an EMD switcher hood on the long end, retaining the Alco short hood. The UP took over the portion of the Bramberger to Hill AFB in 1959 and the 570 was repainted to UP 1270 and based in Omaha. It was traded to EMD in 1972.

In my opinion, the Alco RS2 had the worst fuel filler location of any locomotive design, EVER. It's located on the cab sidewall, and you can see the fuel spill running through the word WEST. The fuel tank was located beneath the cab, also not a good plan IMO. On the other hand, I'm impressed that the UP added a nice all-weather cab window, and it appears that someone took the time to block off the hood top cab windows with brown paper.