By 1950, Alco's (Model) 539-engined switcher line had seen 10 years of production. Over this period, only minor changes had been made during production. Among those previously noted were the changes to the radiator shutter assemblies and the addition of exhaust stacks to better prevent exhaust emissions from entering operating cabs. As a final note to Alco's 1950 production line, the switcher line received a modest set of upgrades. The most important was the replacement pf the Blunt truck with the GSC rigid bolster truck, used by all the other builders in their switcher products. In 1949 the AAR adopted this truck as Recommended Practice. In response, Alco followed suit; the 660 hp switcher was designated E1530A, S-3, and the 1000 hp was designed the E1540A, S-4. In addition to the truck change, the S-3 and S-4 body construction was modified to maximize welded assembly and to avoid the use of rivets where possible. Also, rubber moldings were applied to all windows. The end result was a tighter body with less tendency to rattle.