I have checked and cleaned tracks, wheels, and sliders. It used to be fine. I have one add on set, 43461 B/C. The trouble seems to happen when I add it to the train. It is coupled correctly to the A front unit. I first noticed it moving slightly, then going dead. Yesterday I added it back on, and it was fine. I didn't run it long. Today, it is totally dead. I cant reverse the direction to get it to power on at all. The 2 year warranty expired April 28th. I bought it new from Lippe Detmold.
It has derailed on a few occasions at low speed and tripped the CS3 into stop, but I usually hit the stop bar before it faults.
I appreciate any input.
I have also checked using the other side of the CS3 controller, and with another train. No other trains affected. The supplimental car set is still under warranty. I bought it from MicroMacroMundo in Miami.
I just took the add on car set off, and it's working perfect in both directions. I guess this rules out the decoder.???

Yes, Mr.marklin, it is, and you're right. Each unit has a letter on the underside. I connected the b and c set back on yesterday afternoon and it ran fine in both directions until a derailment. After getting the bogie back on the track, the A front unit had no marker lights, and the interior lights and destination boards out too. Upon reversing direction, the entire train went dead again. I just shut down the system and quit. Maybe a short has affected a part of the decoder?? It's like the reversing signal isn't getting through to the end unit. I hear the click before the lights go out. I can only presume that the reversing relay is integral in the decoder.