I have a Chedder Puffin Verticle two cylinder steam engine and one servo is all that is needed to control both direction and speed. Speed in both forward and reverse. The way they do it is by meens of little ports in the engine. That is all I know about it. If you really want to control speed and direction for hours at a time then go electric. Clean, quiet, and smoooooooth.[sm=wink_smile.gif]