Hi John,
This was one of the topics we covered in my seminar on accessory wiring at the MTH booth at York. Most tinplate crossing gates and signals are simple ON/OFF single circuit accessories. For this type of accessory you don't need a relay, just an insulated rail as Curt described earlier.
Some accessories (mostly modern scale accessories) require one circuit to be powered when a train is present, and a different circuit when a train is absent. These 2-circuit accessories do require a relay run off either an insulated section (my preference), an ITAD, or the old fashion pressure TAD's. I like using the Z-stuff DZ-1008a relay and an insulated rail for many such applications. They're fairly inexpensive, easy to power on most 3-rail layouts, and come with pigtails that make them easy to wire up.
Hope that helps.
