Thanks, Gary. Yup. That's it. From the close-ups I was better able to spot the logos, satellite dome and window blanking in the frame grab. They must have been scooting down the track at a really good clip - 40-50 mph - since it was in only that one frame.
Other geometry cars are immediately obvious as to what they are - NS's and UP's, for instance - and don't move about the RRs by themselves. Don't know the story behind NS's (is it self-powered, or not?), but a few years ago UP had a big row between the engineering and operating departments, and now can't move theirs without a train and full crew.