Trane Air Handler GAT2A0C48S41SAA Create "C" Wire
Inside the unit, you should find a transformer. On the secondary side of the transformer (low voltage side) you should see a red wire attached to one terminal, and blue attached to the other.  The terminal with the blue wire will also have a green with yellow strip wire, which attaches to the chassis.  The blue wire is the C wire.




Using a multimeter set to AC volts, you should measure about 24 volts between the red and blue wires.
Connect a wire between the blue wire terminal on the transformer, and the C terminal on the thermostat.  That should get you up and running.