That track was an industrial track, and I think at one point (well before the sewage treatment plant and I-295) circled around the peninsula to connect to Grand Trunk. It ran along the base of Munjoy Hill before ending, and I think it's now a walking path. Bits and pieces of track still exist under pavement between the track that you see by I-295 and Munjoy Hill. It's not connected to the main line.
I agree that selecting #685 for the $5 promotion seems strange. #685 is the only DE that the half fare senior discount does not apply.
The conductors must be moving the #685 passengers "smartly" on and off tonight with the sold out train is running on time.