Or just use a MOSFET if the relay isn't very fussy. Or, use a combination of the Arduino driving something like the above timer relay, which itself will pull the 12V relay high if that's all it needs.

When the button is released, a pulse of current peaking at 12V is applied to the coil of K1. This turns on the relay, and it remains on after the capacitor has discharged down to the bias voltage, due ...

Firecrown illustrator Kellie Jaeger provides the information needed to wire a Lionel No. 450 Operating Signal Bridge using AC relays. The best to use are single-pole double-throw types.