Reversing Loops




OPERATION - Reversing Loops


In the running of trains, eventually there is a need to reverse the direction of travel. The desire is to send a powered unit into a loop and have it continue on its course in a forward direction, but come out of the loop going in the opposite direction.

To do this, somewhere in the process the power polarity has to be changed in order to change direction. This requires continuity disconnects and reconnects with a different polarity. In doing these things there is always the potential for conflicts in the flow of electrons and a sizeable voltage and or current differential can develop.

The Reversing Loops on our layout are intended to create a neutral area that is not Controller dependent so that polarity can be changed within a limited section of track.

The entry and exit points are the potential source of relatively large surges and so the operation must be undertaken in a way that minimizes the potential of having those surges run through the powered units and the electronic components that accompany them. When you consider that the voltage can be +15 volts on one side of the gap and 15 volts on the other side of the gap, you could encounter a 30 volt differential, enough to cause pitting of the wheels and/or damage to electronic components in the powered unit.

The following definitions are used in establishing the best methods for Reversing Loop switching:

1.The ENTRANCE BLOCK is the block just before the Reversing Loop and is controlled for both power and direction by the hand controller.

2. The REVERSING LOOP BLOCK is the area separated from the rest of the track by gaps in both tracks, so that polarity changes can take place. It is controlled for power by the hand controller, but direction is controlled by panel switches. The direction control on the hand controllers have NO effect in the reversing loop.

3. The EXIT BLOCK is the block just after the Reversing Loop and is controlled for both power and direction by the hand controller.

4. The AUTOMATIC REVERSER is an electronic device wired into the Reversing Loop and which automatically senses short circuits at the Entrance Block and Exit Block. The polarity is flipped within the Reversing Loop as needed to allow locomotives or other rolling stock with current pickup to pass through the loop. Automatic Reversers are necessary when the layout is operating in DCC mode since the DCC cab controller does not affect the polarity anywhere on the layout.

The most hazardous points for surges occur at the gaps between the Entrance Block and the Reversing Loop, and the Reversing Loop and the Exit Block. The power color and direction in both the Entrance Block and the Exit Block need to match the Reversing loop as the powered unit crosses the gap.

There are three ways to do this :

1. Hand Controller Method (DC only):

  1. Set the Entrance Block, the Reversing Loop Block , and the Exit Block power color all the same.
  2. Use the same color hand controller, (matching the power color), for Entrance Block, Exit Block and Reversing Loop Block power and direction control.
  3. While the powered unit is in the Reversing Loop Block, change the direction switch on the hand controller before the powered unit crosses the gap to the Exit Block.

2. Reverse Loop Method (DC only):

  1. Set the Entrance Block and the Reversing Loop Block to the same power color, and the same direction.
  2. Set the Exit Block power color to a different color and the direction switch to the opposite direction of the Entrance Block.
  3. While the powered unit is in the Reversing Loop Block, change the power color for the Reversing Loop Block (on the control panel) to match the Exit Block power color, before the powered unit crosses the gap into the Exit Block.

3. Automatic Method (DCC only):

  1. If the entire train fits within the physical Reverse Loop Block, simply let the automatic reverser take care of the job.
  2. Problems occur if many cars in a train are equipped with current pickup, as is the case for lighted passenger cars. The automatic reverser therefore can not match the Entrance Block and Exit Block polarity at the same time, and it can (usually will) occur where the DCC system shorts and cannot recover.
  3. A work around is once the powered unit(s) enter the Reverse Loop Block, manually shut off the Entrance Block(s) to electrically isolate the remainder of the train. After the entire train has traversed the Reverse Loop, turn back on all blocks for the next operator.

Reversing Loop physical boundary identifications (Mainline operation) all proceding counter-clockwise

CODE B - Garcia (West Loop) from before the bridge to just before the tunnel entrance.

CODE C - Tijuana (South Loop) from just before the entrance to the tunnel by the window to just before the exit from the tunnel in front of the bridge.

CODE D - El Centro (East Loop) - from the farm house by the wall to the switch past the silos.

CODE E - Rose Canyon (North Loop) from the beginning of the draw-bridge crossing to the exit of the draw-bridge crossing going in the opposite direction.

NOTE: All power and direction switching must be done while the powered unit is in the Reversing Loop before it crosses the gap exiting the Reversing Loop for any method.

BE SURE TO RESET SWITCHES AFTER EXITING FOR THE NEXT TRAIN COMING IN.

SAN DIEGO MODEL RAILROAD ASSOCIATION Mainline Reverse Loop Configurations. Each Configuration shows 4 Block Gaps numbered 1,2,3, &4. Blocks are 1 to 2 (Entrance), 2 to 3 (Rev Loop), & 3 to 4 (Exit).

Note: ALL power and direction switching must be done while the Powered Units are in the Reversing Loop, before it crosses the gap exiting the Reversing Loop Block for any method.



 

 

Willard Buxton, September 2001.