I put directional lighting in one of my Athearn (older style - not RTR) GP38-2 this afternoon mostly as an experiment. I had a theory that because of the reduced voltage available to the motor that the Locomotive would run much smoother with one of my Proto2K locos. It turned out that this diesel runs almost the exact same speed as my P2K SD60 and the directional lighting set up looks much, much better. The Athearn still runs fast than my P2K GP30, but that loco runs very slow.
I almost seems that the change makes the engine run a little more smooth than it did before.
I used this directional lighting plan: http://www.mrollins.com/constant.html
Thought you may be interested.
Greg