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