You can always look into a voltage step-up converter. Pololu makes some that are small are easy to use. I had to use a 3.7 to 12 volt converter for my battery powered locomotive. The battery only puts out 3.7 volts but the LM needs 12-14. Works like a charm.
For your situation, you could put one on the output wires going to the couplers. As far as I know that should work.
-Kevin