In order for the Auto Reverse Controller (ARC) to sense when an engine or trolley reaches the end of the track an insulated section of track must be installed near each end.  When the train enters this section is stops and the ARC senses a drop in voltage and knows where the train is.

In order to get the train out of the insulated section a diode is placed across the cut in the track.  Diodes only conduct in one direction.  When the train enters the diode section it comes to a complete stop.  When the ARC reverses the polarity to bring the train out of the section the diode conducts and the train operates again.

Diodes are available at Radio Shack.  Make sure you get ones with a high enough rating to handle the power drawn by your engine.  A small engine will work well with a 1 amp diode.  Larger motors in larger engines may need diodes with a 2 or more amp ratings.

RADIO SHACK ITEMS:

1N4003 Micro 1-Amp Diode 
$0.69
Catalog #: 276-1102

  

4 Rectifier Diodes  6-Amp
$2.49
Catalog #: 276-1661