Ted,
Riding on a weekend, you may have the chance to see a second train in operation on the line. The GLRR is running an Extra Train on busy weekends (which has proven to be all weekends since the extra train was started in late June). The regular passenger train retains its schedule, and passes the extra at Hall Tunnel Siding. You may have the opportunity to witness 2 engines under steam on the Loop, as well as the passing procedures of the RR.
As far as a cab ride, you can ask one of the train crew or engine crew. If there is room in the cab and it is ok with the engine crew, you may be able to get a ride in the cab. The Loop does not solicit cab rides, but the employees are friendly and usually don't mind a couple of guests, granted that they don't get in the way too much!
Shane
GLRR volunteer, Extra Train