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Great question AC!!...idk?
....I just hooked up an eight car load to see, and the bridge wont retract as long as cars are passing over it!!! IDK how it works, (not going to dissect it to look inside), but to answer your question:The number of cars is unlimited!!

I suspect its a bit of a dead mans switch, the train rolling over puts weight on the bridge, once that weight is gone the bridge is able to move up slightly and retract. Without having one on hand though its hard to say.