i just purchased a bachmann empire builder train set. It runs great except i can't run the 2 passenger cars at the same time because they come un-coupled. I can run one at a time with the a & b unit and it runs fine. It doesn't make any difference which passenger car I run as long as i don't run both of them at teh same time. But when i try to run both passenger cars at the same time they come uncoupled. thanks for any help you can give me.