How to wire 213 Lift Bridgee

Here's a different wiring diagram from Lionel Supplement # 47 showing what all the connections are for on the 213 Lift Bridge (click to expand image):

When discussing how to wire an accessory, I generally avoid using A and U post references because some transformers have U as Common while other transformers use different letters as Common.  Also, Common is sometimes referred to as Ground, but I prefer the term Common because it's electrically isolated from earth ground.  The Hot and Common connections on transformer outputs have opposite AC voltage polarity.

If providing power to the bridge from an accessory power source, that accessory transformer will need to be Phased with the other transformer which is providing power to the tracks leading up to the bridge.  The two transformers' Commons will then need to be connected together after they are in phase with each other.

Once that is accomplished, the Hot connection from the accessory transformer will need to be connected to the single "Track Power Connection" on the bridge base; indicated by the red arrow in the diagram above. The shared Common connection from both transformers passes through the outside rails to the bridge track, since only the bridge center rail needs to be insulated.

Both the Hot and Common from the accessory transformer will also connect to the 2 "Power Connections" on the base located at the other end of the bridge.  These connections provide power to the bridge electronics, which feed the lift motor and lights.  Polarity to these last 2 connections doesn't matter because this part of the circuit only feeds the bridge's internal circuitry, where it's rectified to DC voltage to drive the motor and LEDs.

Thank you.