The right-hand rail is positive when going forward looking at the locomotive as if you are the engineer sitting in the right-hand seat.
cedarwoodronI am fixing up an old Athearn SW7 calf and am rewiring the headlight for an LED. If the calf is being backed up, so that the headlight is going forward (without freight cars connected), which side of the truck should be wired for "forward" motion? If I am looking down on the top of the hood with the headlight aligned with my forehead, would it be left or right? (this would be where I am running the power unit cow in reverse) Cedarwoodron
I guess the question really is which end of the calf is the front. Looking at the HOSeeker site, Athearn seems to have assembled the model the same as the cow. That is, the sloped non-cab end is the rear. See http://www.hoseeker.org/assemblyexplosionAthearn/Assembly%20Explosion%20Athearn%20SW1500%201973.jpg
But looking at some random web photos it appears the the cab end of the cow and the sloped end of the cow ran back to back. See http://www.northeast.railfan.net/images/brc503a.jpg, http://www.northeast.railfan.net/images/brc506.jpg, and http://www.northeast.railfan.net/images/up1875.jpg.
I think if you install the trucks as shown on the Athearn diagram, they will be on the proper side when you turn the unit around to mate it with the cow. This would make the normal forward direction of the calf opposite the normal forward direction of the cow, and the calf headlight would illuminate as it moved toward the disconnected freight cars.