I think you answered the question yourself when you said some tenders are left rail pickup and som are right rail pickup. Prior to the NMRA< there really was no standardization of anything. You'd think that a given manufacturer would amke all their locos the same, but it was also a time of rapid advances and what worked well on one model, when they came out with a new one a year or two later, they may have found something else worked better.
I'm not sure it even matters. DCC doesn't care. The track pickups can be hooked up any old way, it's the motor drive leads that matter to make sure the loco moved forward when forward is selected on the throttle. It matters in DC only so far as that when the right rail is positive, it's connected to whichever motor terminal makes the motor spin in a way that the loco moves forward. There hav ebeen locos sold even AFTER the NMRA came along that were wired backwards.
And nothing says the opposite rail can't be the frame ground in a BB loco - just swap the front and rear trucks around, now the left rail is the fram instead of the right rail. And it will run the opposite way, unless you flip the motor over. It's a way to fix the shorting through the couplers, but a rather extreme one - if you use metal couplers in the metal pockets on a BB loco, and you flip one around so they run back to back, they will already both move in the right direction, but on one loco, the frame is the left rail, and the other, the frame is the right rail, so you get a short through the coupler. The more sensibe fix it to either use one of the plastic chank couplers, or grind the mounting area flat and use the Kadee box (plastic) to mount the coupler in.
--Randy
Modeling the Reading Railroad in the 1950's
Visit my web site at www.readingeastpenn.com for construction updates, DCC Info, and more.
RR_MelBefore NMRA Specs was there a manufactures standard on which rail was the tender?
RR_MelAll the old Athearn frames are connected to the right rail positive forward but early Mantua and some of the AHM locomotives have the frame connected to the left rail.
If the frames are connected differently, then the motors must be connected differently. The goal is to have have all the locos move in he same direction, back when there were only DC controllers. How that would have been phrased is a touch beyond my paygrade.
Henry
COB Potomac & Northern
Shenandoah Valley
For DC operation, the NMRA standard was right hand rail positive was forward. I'd guess most often it was the lower motor brush that was grounded to the loco frame, with the wire coming off the top one, but there was nothing that said it had to be that way.