…X marks the spot.
I'll bet that at some point in your T-Trak adventures, you saw the Unitrack 20-210 Double Crossover in Kato's catalog and thought to yourself, "double track, 310 millimeters long…all I need to do is fasten it to a module box, and I'm done! Just one piece of track! Easiest module ever!"
Not so fast, there. Take a look at the instructions, and notice that there are insulated gaps in the inside rails of both tracks. In other words, the double crossover does not carry track power from one end to the other. If the rest of your layout is a simple oval, one set of feeders will suffice to get power around the layout—that is, until a rail joiner fails and leaves half the layout dead (which is exactly what happened to us at the November '11 Syracuse show). For reliable operation, you really need to ensure that power is fed to both sides of the crossover. How? You can take one of three approaches:
Use two feeder modules. Plan your layout to have a feeder-equipped module on both sides of the crossover, and wire feeders from each track together at the power supplies. For all but the smallest layouts, this is a good strategy anyway.
Equip the crossover module with jumpers. Build it as a double-length module, with straight track at either end. Use terminal joiners to attach jumper wires across the crossover for carrying track power.
Modify your crossover internally. If you are brave, you can open up your crossover and add jumpers inside which will allow you to use the unit on a single without feeder wires or jumpers.
Okay, so much for power. How about actually running trains over it? If both lines are operating on DCC, it's easy, just throw the turnout levers and run the train over to the other main. With both lines operating on conventional DC throttles, heed Kato's instruction: "…the direction and speed controls of both power packs should be matched. Remember you are crossing to another line." If you've operated layouts with two connected loops, or an Ntrak layout with a crossover module, you know the drill. As your train crosses over, it receives power from a different source. If the polarities do not match, it will cause a short and stop dead at the gap. If voltages aren't close, it will lurch.
At recent shows, our club has been running the outer line on DCC, with the inner loop(s) connected to DC power packs. The presence of a double crossover module in this situation, even for purely decorative purposes, is asking for trouble, so we'll simply leave it out. Accidentally smoking a power supply is not my idea of fun!
Postscript: check out this video of a crossover module being put to interesting use in a T-Trak layout!