Yeah, that's prtty much it. Now, when you subject something paramagnetic like steel to a magnetic field, you can create a temporary magnetic field in that object. The presence of the paramagnetic material reshapes the magnetic field, so if you have a bunch of cars in close proximity to the metal floor of the box, the floor may have a magnetic field emanating from it. That field, over time, could disrupt the orientation of the dipoles in the cars' magnets. But this is a very weak field, the cars aren't all that close to it, and improvements in magnets make them less susceptible to having their dipoles 'unaligned'.
But yeah, what you mentioned earlier about folks with pacemakers is true, and a steel box does act as a shield. One problem, though, is pacemakers often have trouble dealing with the electrical noise generated by small motors. So when your car comes out of your nicely shielded box, and is put on the track (especially after you've cut away the little electronic thing on the motor for tuning purposes), folks with pacemakers can really be at risk. We lost a club member, a really great guy, because his doctor told him he had to stay away from these little motors. They not only have strong magnets in them, but they also generate rapidly fluctuating magnetic and electrical fields that pacemakers don't like. Jim is still alive, but we don't get to enjoy his company as often as we'd like.
Sorry for all the 'Mister Science' stuff, I know it's a long read.