Lionel RS-3 LionChief Plus runs w/1 truck on track shorts w/both trucks

Yes, look closely at wiring and terminals at the offending truck.  G

Limited space on a truck, with the add of additional third rail contact pic-up rollers, a short could easily develop. 

 Atlas SW truck.  Most of the black is common, associated with the  bottom axle, and wheels. The Third rail pick-up is mounted to a plastic insert, with the correct length screw.  

Because it works OK with either truck on the rails but not both I'd look under the hood to see if the ground (outside rails) and center rail wires are reversed with respect to one truck vs the other.

Have you tried removing the pick-up from one set of trucks and seeing if the train runs properly on the track ?

Hi Richie l I removed the black wire w/white stripe from the pick-up. Still shorts out . I than took the blue wire off the base of the motor (Which I thought was ground) with both wires removed, engine runs properly on the track. minus 1 pick-up & whatever the blue wire does.

geysergazer  Richie C   GGG  I believe  what geysergazer posted to check the wiring. 1 pickup wires and 1  ground wire are tied together which to me doesn't make sense. Without a schematic, I'm going to solder the 2 pick-up wires together w/black wire going to the board. And the 2 ground wires together w/ a brown wire going to the board.  If any one thinks this is wrong let me know before I have to replace a board. Thanks to all for the help.

I think that's it. Which wire set; roller or ground; goes to which doesn't matter too much with ac

Using a meter set to ohm/continuity you might insure one board lead or the other doesn't tie directly to chassis ground elsewhere.

It does if they are mixed up at the board and it also matters for bell whistle function.  G