Phil,
There were two types of transmissions applied to the S2. The catalog image I posted earlier was the first generation. It has a large cast gearbox, a very large pulley, and a separate idler pulley. The second generation transmission is smaller and it pivots on the axle to tension the belt.
In the case of the SVRR car in the first photo, similar to the older EBT cars, the engine is mounted on a sliding base which predates the idler pulley arrangement. I'm pretty sure that's a transmission hiding behind the large rear pulley. I agree though that I can't see the shift lever.
There may be no shift lever above the deck as I'm not sure that this model of transmission was meant to be shifted on the fly. Basically the setting was chosen before departure. The only other thing that comes to mind may be an early version of Fairmont's Hy-Drive. This arrangement used a chain reduction drive to raise the axle pulley up to clear debris between the rails.
Inquiring minds would like to know more.
-Jeff