Left and right are as though you were in the cab. It doesn't matter as far as the wiring from the decoder to the pickups, it's ambidextrous anyway. The only thing that matters is the the brushes on the motor, and if you get that wrong you can flip them. In theory, hooking the orange wire from the decoder to the brush that used to be connected to the right rail should make "forward" "forward".
Jeff But it's a dry heat!
Need information on installing decoder (DCC) in old engines, such as AHM or Rivarossi.
How do you determine left side or ride side of engine (do you consider yourself inside the cab or in front of engine facing you).