Two rail model railways have a problem when the track is wired to form a reverse loop as shown below. The outer rails join together to give a short circuit. The short circuit can be prevented by putting four breaks in the rails. Two breaks would solve the short circuit until an engine ran across the break. It would end up with the wheels bridging the gap and a short circuit would still occur.
Automatic running around a reverse loop requires two things. The points to change and the track polarity to change
The diagram shows 4 isolation breaks in the track on the reverse loop. Whilst the train is travelling around the loop the feeds to the straight section need to be reversed. There are several ways to accomplish this.
Feed the controller into the straight (approach track) and connect to the reverse loop through a double pole double throw switch wired to reverse the polarity. This switch needs to be thrown whilst the train is on the reverse loop so the straight track is ready for the change of direction. The snags are that the train needs to be stopped whilst both the direction on the controller and the switch are operated and the switch must be left in the correct direction for whichever way the next train is to travel around the loop This would work for both DC and DCC..
Feed the controller directly to the straight track, but feed the controller into the reverse loop through a diode bridge. The diode bridge will cause the train always to travel in the same direction around the reverse loop regardless of the controllers direction setting. The snags are the diode bridge does not work with feed back controllers. 1.2 volts are lost through the diode bridge slightly affecting the trains speed. This method does not work with DCC. You need to remember to reverse the direction with the controller but there is no switch to throw.
A different method which is suited to where there are reverse loops at both ends of the line is to switch the polarity of the straight section of track and feed the controller directly into the reverse loops. If ais used in place of the switch the direction reversal will be automatic. To trigger the latching relay board the "P" terminals of the IRDOT-Ps can be used. This is shown for two reverse loops. Only two IRDOT-Ps are required for both reverse loops. Each IRDOT-P switches both points. They set the correct direction for leaving the loop the train is on and they set the direction for entering the other loop.
The following diagram shows the wiring of the latching relay board in detail. The green and red lines are the power from the controller to the track after passing through the relay contacts. The "P" terminals of the IRDOT-P are connected to "S" or "R" when the train is detected by the IRDOT-P. This connection makes the relays latch on or off, so setting the direction for the straight line. This arrangement gives completely automatic operation of the reverse loops. Electrically the contacts on the Latching Relay Board have been wired to reverse the polarity in exactly the same way as the DPDT switch was. The two boxes labelled "PB" represent pushbuttons. Their use is optional. They allow the direction to be changed manually on the straight track to enable shunting. Pressing one push button sets the direction to the left the other to the right. This allows the controller to be left set at the correct direction for the reverse loops. A further refinement is to wire two LEDs to indicate the direction set by the latching relay board. A second latching relay is used for this. "R" of the first relay is wired to "R" of the second, and "S" of the first relay to "S" of the second. This will cause both relays to be operated by the IRDOT-P or push button switch.
The following video made by Mr Carl Roberts of his excellent Rowland Emett cartoon inspired railway uses this method.
It is also possible to have automatic reverse loops wired to operate for trains to travel around them in either direction.
Although isolation breaks are still required as already explained and the points still need switching the polarity reversal can be dealt with differently. The reason for this is that analogue has dc electric motors powered by a dc voltage on the rails. The direction dc electric motors turn and thus the direction the train travels depends on which rail is positive and which rail is negative. If you reverse the polarity of the rails the train will travel in the opposite direction. In contrast dcc has an alternating voltage which provides both the power and the engines speed and direction information. The direction information is that the train is to travel forwards or backwards.
Another way to look at this is if you have a dc engine travelling to the left and you pick it up turn it through 180 degrees and put it back down on the rails it will still travel to the left. If you have a dcc engine travelling to the left pick it up turn it through 180 degrees and place it on the rails it will now travel to the right.
The result of this is that for dcc the polarity of the rails on the reverse loop does not matter. Unfortunately the meeting of two different polarities at the insulation breaks does matter as as soon as the (pick up) wheels bridge the gap there will be a short circuit causing the dcc power to switch off. DCC manufacturers make polarity reversal devices which detect the short to switch the polarity. They have to detect it quicker than the dcc power source which will turn the power off if it detects it first. Usually they do but not always depending on the dcc controllers design. A different solution is to use IRDOT-3D to reverse the polarity before the insulation break is reached.
As a train leaves a reverse loop the point needs to switch at the correct time to prevent a derailment. If you are using solenoid point motors (peco, hornby, Seep)An IRDOT-P is ideal for doing this automatically. The IRDOT-P at 1 is positioned to operate the points just before the train reaches them, ensuring the rear of the train is clear of the points. The IRDOT-P at 2 sets the points for trains approaching the reverse loop. It is positioned more than a train length from the point. This position is to prevent the train being derailed as it re-crosses the IRDOT-P at 2 on leaving the reverse loop. If the IRDOT-P at 2 needs to be positioned closer than this the Direction Detector can be wired to the "I" terminal of the IRDOT-P at 2 so that the point is not switched by right to left trains. However the Direction Detector will only work with dc (analogue) model railways. If the polarity on the reverse loops is switched automatically then this arrangement will allow the trains to run automatically around the reverse loop but they will always run in the same direction, clockwise in this example.
By positioning both the IRDOT-Ps on the reverse loop then the train will be able to travel the reverse loop in either direction. A manual method of switching the point can be wired to it in addition to the IRDOT-Ps if you wish to select the direction. Without this switch the trains will simply alternate between travelling clockwise and anticlockwise.
For slow motion point motors IRDOT-3Ds can be used in a similar way to the IRDOT-Ps.