The S-6 and T-6 are two very different animals. The S-6 was designed as a yard engine, whereas the T-6 was designed for higher speed running on mainlines. This is probably why the T-6 sported the more convenient sand filling end notches as seen on the RS-11s and C420s, instead of the rooftop hatches on the S series.
S series- Switcher, RS series- Road Switcher, T series- Transfer.