If you mean that you have a bridge that lifts out entirely, and is then lowered into position, you could use gravity and have the pointy ends of bronze nails (wired to the bridge track section) on the under side of the bridge, that come into contact with 1" by 1" bronze plates on the surface of the bridge supports (wired into the main track supply). You could make a little "shelf" at both ends, from a piece of 2 x 4 lumber, that the bridge sits on, and where the contact plates are mounted.
Is there a reason that you don't want to have a cord that requires a manual connection? It would seem more prudent to have a system that requires a manual act to make the connection, to ensure that everything is ready to roll, before jeopardizing the little people on your trains.
A cord can have a couple of extra leads, wired through a loop connection on the bridge, that activate a fail-safe relay, which cuts off track power in advance of the bridge a few feet in both directions if the bridge is out.
