Hi Whit, from the Lionel supplements, it shows that these cars have the 9551-80 operating coupler trucks, but I can't tell if they both have pickup rollers for the center rails.
update, a quick search of the internet shows that only one of the trucks have a pick-up, so that is the reason for the flicker, if power is coming from both trucks, then the power would come from two sources. It looks like you could add another pickup roller to each car, the pickup usually just snap in. OGR had an article a few years ago that showed someone basically wiring all of the cars together.
