Track wiring for control and operation of D.C. layouts

Track wiring is the means by which we supply electrical current to the motors which power our locomotives. The most common power source is a variable 12 volt D.C. supply with a switch to control train direction. Model locomotives are usually manufactured so that when positive (+) voltage is applied to the right side wheels, the loco will move forward. It is up to the modeler to define which way is forward and then design his wiring to fit his layout. The discussion here will deal with pure D.C. voltage only. D.C.C. (Digital Command Control) will be addressed in a separate chapter.

It is always a good idea to make a schematic diagram to illustrate how the track wiring will be accomplished and where the wire runs will be located. Notations on the diagram will aid in installation and possible trouble areas when actual wiring begins. Problem areas such as turnouts, crossings, and reverse loops will require special treatment and isolation gaps to prevent short circuits. Plan your wire runs for ease of access and neatness for maintenance and future wiring changes. Wire nuts are fine when twisted connections of 2 or more wires of the same kind are used. However, connections of different wire gauges and a transition from solid to stranded wire should be soldered since wire nuts do not work as well in these instances. Poor and improperly soldered connections are the most common and hard-to-find problems associated with model railroad layouts. Special care should be taken to insure that all wires are properly insulated and positioned to avoid the possibility of shorts. Terminal strips should be used at both ends of wire bundles to properly separate and identify wires branching to different locations.

For larger layouts,track wiring can quickly become complicated and should be carefully planned and installed in a neat and orderly manner. Wire can be purchased in single or multi-conductor rolls with colored insulation jackets. Create a color code and mark wire connections at source and termination with numbered labels for identification. If you group runs and use cable ties, be sure to test everything before you tie the bundles together. Nothing is more frustrating than having to cut cable ties and separate and trace wires back to terminal blocks to identify them. (I speak from experience here!) There seems to be a standard rule governing model train builders that problems always occur in the places most difficult to reach.

The speed of a model locomotive is controlled by varying the D.C. voltage applied to the rails of the track. This voltage causes electrical current to flow through the wheels of the loco to the motor causing it to rotate and subsequently move the engine. Direction is controlled by a switch which changes voltage polarity to the rails and determines whether the train moves forward or backwards. It should be noted that when an engine that is moving forward is lifted and rotated 180 degrees and placed back on the rails it will continue in the same direction but now it is moving in reverse. This effective polarity reversal is overcome in D.C.C. equipped locos which move forward even if you turn them as stated above.

D.C. Controlle

Solid wire should be attached to the rails as feeder wires (#18 gauge solid) and routed through holes drilled in the roadbed. It is important to clean the rail thoroughly where the wire will be soldered. Use a small file or sandpaper to polish the area and apply a small amount of soldering paste to the side of the rail. Apply the soldering iron tip to the area and carefully tin the outside of each rail where the feeder wires will be attached. Strip the insulation from a length of solid wire and feed it through the holes on the outside of both rails. Make a 90 degree bend about 1/8” from the end of the wire and tin this end. Place the wire against the tinned part of the rail and use a block of wood or some other weight to hold the wire against the rail and ensure that firm contact is established between wire and rail. Heat the junction and make certain that the solder flows around and through the joint.

Rail Wir

The feeder wires can now be attached to the positive and negative wires from the controller track supply. Wires soldered to the rails are critical for proper operation and can be a source of many headaches if not done properly. Be sure to spend a little extra time on this chore and use the right soldering iron and follow the proper soldering technique. The simplest track wiring arrangement for a train layout is the continuous loop as shown below.

Loop Wiring

The positive (+) track voltage from the controller is wired to the outside rail and the negative (-) is wired to the inside rail. When the DPDT switch is in the position shown, the loco will move forward. When the switch is thrown, the polarity is reversed and the loco will move in reverse. For this layout the track wiring is simple and fairly easy to do. Most controllers will have a direction switch as an integral part of the unit-It is shown here only for the purpose of illustration and the fact that more complicated track wiring schemes will require external switches.

Most modelers will eventually tire of running trains around and around a loop and want to add more interesting operation.

Loop Wiring

This example shows a more complicated layout by the addition of two turnouts and a return loop. Return loops are used to turn locomotives, even entire trains if the loop is long enough, and are handy track arrangements to have but do require some special track wiring. Loops such as this should be completely isolated from other tracks using rail gaps to prevent short circuits as shown above. Gaps can be cut with a rotary tool or razor saw and filled with epoxy, plastic, or insulated rail joiners. Check to be certain that the inside of the rails are smooth and file if necessary. Operation proceeds as follows: Turnout#1 should be in the closed or straight position-turnout#2 should be in the thrown or diverging position. Main and loop switches are set to match polarity on both sections. Once a train is completely on the loop, the main switch is thrown reversing the track polarity which now matches the loop polarity and turnout #1 is thrown to route the train back onto the main. The train is now traveling in the opposite direction on the main loop. If turnout #1 and turnout #2 are aligned for the train to travel the main loop in the opposite direction, the loop switch must be thrown to reverse the polarity on its rails. Notice that now the inner rail is positive and the outer rail is negative. Since the train is moving in the opposite direction the positive rail is on the right side of the loco and the engine is moving forward.

Another track wiring arrangement which is often used to turn locomotives and trains is the wye.

Wye 1


The two drawings show the wiring and gap arrangements for both insulated and all-rail turnouts. Although there are slight differences between the two methods, the operation is the same and is as follows: A train going forward passes through turnout#3 and turnout#1 and stops after clearing the switch. Turnout#1 is thrown and aligned with the left leg of the wye. The branch DPDT switch is thrown reversing the train direction and matching the polarity of the branch with the left leg. The train backs through turnout#2 and stops clear of the points and turnout#2 is closed aligned to the main track. The main DPDT switch is thrown, reversing polarity, and the train moves forward and in the opposite direction from its original path. The safest procedure for wiring loops and wyes is to check each turnout as it is installed by running a locomotive forward and backward through each leg before moving to the next step. It is much easier to identify a problem in this manner than to completely wire the array and then try to locate a malfunction.

Cab control and Block wiring

For a D.C. wired layout the only way to run multiple trains simultaneously is to use independently controlled, isolated track blocks wired in a configuration using separate controllers for each locomotive called Cab Control. The easiest way to describe this system is to use the drawing below and follow the operation.

Block Ca

Although the system is compressed for the purpose of illustration, the drawing shows 5 blocks separated by rail gaps and controlled by 2 controllers (cabs) which can be selected by the block switches for each section. As shown above, blocks 1 and 2 are set for controller A and blocks 3, 4, and 5 are set for controller B. The direction switches for both controllers are set for their respective trains to move forward. Train A is moving westbound on the main and is in block 2 approaching block 3. Train B, which was originally on the mainline, has been switched to the passing siding and is proceeding eastbound in block 5. For Train A to continue on the main, block 3 and block 4 must be switched to controller A thereby switching polarity on their rails and control of the direction and speed of Train A to that controller. For Train B to continue and eventually switch from the siding to the mainline, block switch 1 must be changed for controller B. If Train A continued across the rail gaps at block 3 with the block switch still set for controller B, the wheels would create a short circuit between block 2 and 3. The same would occur for Train B at the gaps for block 1. Although this sounds like a lot of special wiring and switch throwing, it does eventually become routine for operators who use this system. Block sections can be any length you wish to make them and have the advantage of operation and train control that mimics the real railroads.

Common Rail Track Wiring

Common rail track wiring defines a method which uses one rail as a common, continuous connection to one of the voltage outputs of the controller. So far, we have shown all track wiring as separate positive (+) and negative (-) wiring connections to all rails but it is possible to wire one rail of one polarity to a common source.

Common Rai

In the drawing above, the negative output of the power supply has been chosen as the common connection to the inside rail of both loops. The separation of block 1 from block 2 is provided by the gaps in the turnout and power for each block is selected by SPDT switches 1 and 2. Controllers A and B have their negative sources wired together and all negative rails of this plan are tied together to this common source. Each controller has independent control of the block to which it is connected. As seen, the common rail system is simpler to wire and is popular with a large percentage of model railroaders. It is not recommended for reverse loops (which need to be separate track sections) and is definitely not for use with those who intend to eventually convert to Digital Command Control (D.C.C.)

Passing siding using common-rail wiring

Siding Pas

This illustration shows how to wire a common-rail passing siding. The gaps shown isolate the frogs of both turnouts and allows power to be on or off to the siding through the spst switch no matter which way either turnout is aligned.

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