Anyone know what year MTH went from a 5-volt board to a 3-volt?

2004 can give you either depending on the engine model. After that should be good to go on the 3V.

The premier engines went first in2004. Some rail king in that year still had 5 volt boards. 

Premier vol 1 2004 and Rail king Vol 2 got that straight from MTH.

Chris,

While I can't tell you exactly when MTH switched to a 3V board, 2004 or early 2005 sounds right. When I bought my first MTH Railking engines in the fall of 2005, they both had 3V boards.

Erol Gurcan

1. I contacted MTH on that subject I believe the young lady that replied was the one that moved to Texas I believe her name was Kristen and she was the one that gave message Premier Vol 1 2004 and Railking Vol. 2 2004. 

There was also a little overlap in the transition, I suspect some products that had long production delays had the 5V boards later than others.

PS-2 3V was designed for the #1 gauge and transitioned to the O.  There might be a 2003 item or two that was late production that wound up with the 3V board.  Just as some had the 2 AA batteries vice combined one with molex plug.  There also can be some 2004 with the 5V still depending on timing and production. G