2/10/2015 How to wire a layout for twotrain operation | ModelRailroader.com

http://mrr.trains.com/howto/dccelectrical/2014/06/howtowirealayoutfortwotrainoperation 1/6

Fig. 1 Cabcontrol wiring. This diagram shows a singletrack block controlled by two power packs (cabs) whichmay be selected with a singlepole doublethrow (SPDT)switch.

How to wire a layout for two-train operationDC cab-control wiring works for simple or complex layoutsBy Neil Besougloff | Published: Friday, June 06, 2014

The initial goal of a new model railroader flush withlocomotives, track, and rolling stock is to operate twotrains on his layout at the same time, each withindependent speed and direction control.

This milepost can be reached with cabcontrolwiring. With two direct current power packs, singlepole doublethrow toggle switches, and hardwarestore wire, you can divide any layout into electricalblocks that will allow two trains to be operatedindependently.

How it works

With cabcontrol wiring, a layout using two powerpacks is divided into several electrically isolatedsections called “blocks.” One such block is shown infig. 1. Each block is independent of all the others, so a train in block A can be operated by power pack A, anda train in block B can be run by power pack B.

Each block starts and ends with a plastic insulated rail joiner or a narrow gap cut through the metal rails. Sowhile a cab control layout visually looks like a continuous track, electrically it’s several independent tracksections that line up with one another at each end.

As an operator using power pack A moves his locomotive through one block and approaches the next block,he uses a toggle switch to connect the second block to power pack (cab) A. Now the two blocks areelectrically united to cab A and the locomotive seamlessly moves from the first block to the second. Justbefore he enters a third block, he again connects the new block to cab A. Following this pattern, the operatorcan move his locomotive from one end of a layout to the other.

While this is occurring, a second operator using cab B can operate his locomotive elsewhere on the layout byconnecting the blocks he’s using to cab B. In this manner, the two operators can follow each other around alayout, flipping toggle switches to align the blocks to cabs A or B as needed.

The only downside to cab control is that two locomotives cannot share the same block at the same time.Operator B has to wait until operator A has cleared a block in order to toggle control of the power in that blockfrom cab A to cab B.

2/10/2015 How to wire a layout for twotrain operation | ModelRailroader.com

http://mrr.trains.com/howto/dccelectrical/2014/06/howtowirealayoutfortwotrainoperation 2/6

Fig. 2 Layout blocks. This diagram shows cabcontrolwiring for an oval layout with a passing siding (block 3),four other blocks, and two cabs.

Singlepole doublethrow (SPDT) electrical switches can connect only one cab to a block at a time, so there’sno way for both operators to connect their power packs to the same block at the same time. The only way toget into trouble is to run a train across the insulated rail joiners into another block connected to the other cab,so it’s important for operators to know exactly where one block ends and the next begins.

Blocks and wiring

A layout at minimum needs three blocks for twotrains, but more blocks give operators moreflexibility. Fig. 2 on the next page shows wiring for asimple oval layout with one passing siding that’sbeen divided into five electrical blocks. Each block iscontrolled by its own SPDT toggle switch.

Some SPDT switches have a center off position thatdoesn’t connect power in either direction. This isespecially useful for passing sidings (and spurs),since it allows you to cut power in that track to park atrain.

It’s also smart to turn a block off after your traindeparts, or the next train that enters the block will becontrolled by the wrong cab, leading to the cry “Someone’s got my train!” Each block has a feeder wire connecting it to an SPDT switch. If a layout has six blocks it will need six SPDTswitches.

When making track blocks, you need to insulate or cut a gap in just one rail, not both. Think of a light switch inyour house. When you turn a lamp on or off, you are only interrupting one of the wires that lead to the lamp,not both.

In cabcontrol wiring, the rail that isn’t insulated or cut is called the common rail. This rail gets connected toboth power packs. See the blue wire in fig. 2. While connecting two power packs in this manner may cause abit of anxiety for a model railroader new to electricity, trust us, it works.

Also, by using a common rail, you save on wire, toggle switches, and soldering, since you’ll need to purchaseonly SPDT switches instead of the more expensive doublepole doublethrow (DPDT) switches.

Most hardware stores sell toggle switches, and sometimes a hobby shop will sell electrical components(beyond wire) for model railroads. An alternative to toggle switches are Atlas’ Selectors, which are banks offour SPDT switches designed specifically for model railroads (see the next page), and using them doesn’trequire soldering.

2/10/2015 How to wire a layout for twotrain operation | ModelRailroader.com

http://mrr.trains.com/howto/dccelectrical/2014/06/howtowirealayoutfortwotrainoperation 3/6

Fig. 3 Complex DC block controls. Senior editor JimHediger used miniature toggle switches in this compact DCpanel that controls two reverse loop staging yards and onestubended yard on his HO scale Ohio Southern

Power packs

The easiest way to build a cabcontrol layout is by using a pair of identical DC power packs. Figure 2 showshow to connect one track terminal from each power pack to the common rail on your layout. Keep in mindthat your track configuration may shift the common rail from the inside of your layout to the outside. Followthe rail around your layout with your finger to be sure you have correctly identified the common rail so thatyou don’t break or “gap” it by mistake.

If you’re using two different power packs and mix up your wiring, it’ll create a short circuit and activate thepower packs’ circuit breakers to cut power until the short is fixed. Don’t worry, though. As the breakers coolthey’ll automatically restore the power once the short is fixed.

Simple or complex

Cab control wiring can be as simple as fig. 2, or itcan utilize many more blocks and power packs to letadditional operators connect to a specific group ofblocks on a large layout. The complex, colorcodedcontrol panel shown in fig. 3 operates three differentstaging yards on Jim Hediger’s Ohio Southern.

Cab control of model railroads has been around fordecades, and that’s no surprise – it’s the best way tomove from a single loop layout to more complexoperation, short of entering the world of DigitalCommand Control.

How to wire a reverse loop

Any track configuration that permits a train to change its direction without simply backing up needs aspecial electrical circuit to prevent a short. Wiring a reverse loop, wye, or turntable may seem similar to blockwiring, but there are some important differences.

The illustration shows basic reverseloop wiring for a directcurrent layout. This special situation requiresdoublepole doublethrow (DPDT) toggle switches to change the polarity of the two rails in the reverse loopand in the track block preceding the loop. These DPDT switches allow an engineer to align the polarity of therails so his train may enter or exit the reverse loop without causing a short circuit as it passes over theinsulated rail joints.

If a train enters a track wye to turn around, insulated joints and a similar combination of two DPDT switchesare needed to align the polarity so the train may leave the wye.

2/10/2015 How to wire a layout for twotrain operation | ModelRailroader.com

http://mrr.trains.com/howto/dccelectrical/2014/06/howtowirealayoutfortwotrainoperation 4/6

Basic reverse loop wiring. This diagram shows the wiringneeded to connect a DC reverse loop to a single powerpack.

Atlas Selectors. Three Atlas Selectors are connected sidebyside to control 12 blocks on this small N scale cabcontrol layout.

Combining cabcontrol wiring, reverseloop wiring,and other wiring needs can quickly get complex. Seefig. 3.

For many years Atlas has offered the no. 215Selector, which is an inexpensive set of four singlepole doublethrow (SPDT) slide switches for modelrailroad applications. Each Selector has screwconnections and can be used with two power packsto operate four blocks. Selectors are designed to beused in multiples, so any number can be easilyconnected sidebyside (called ganging) to controlany number of blocks, as shown above.

Like SPDT toggle switches, Selectors have threepositions, a connection to power pack A, a center offposition, and a connection to power pack B. TheSelectors are even labeled A and B.

Atlas also makes a companion device called the no. 220 Connector, which contains three singlepole singlethrow on/off SPST switches in the same housing.

2/10/2015 How to wire a layout for twotrain operation | ModelRailroader.com

http://mrr.trains.com/howto/dccelectrical/2014/06/howtowirealayoutfortwotrainoperation 5/6

3 comments

Added 7 months ago

Most recent | Oldest to newest

Cab control terms

Block: A section of track on a layout that is electrically isolated from adjoining sections by use of plastic railjoiners or gaps cut in rails. Blocks can be any length – usually determined by a specific track plan.

Block switch: A singlepole doublethrow (SPDT) electrical switch that’s used to connect one wire to either oftwo other wires. While most hobbyists think of toggle switches like those shown in figs. 1 and 2, slide androtary switches can also have SPDT contacts. Some SPDT toggle switches are made with a center offposition that does not connect to either wire.

Cab: Another name for a directcurrent power pack with speed and direction controls to operate a train.

Cab control wiring: A method of wiring a model railroad using two or more power packs and track electricalblocks to allow independent operation of two or more trains.

Common rail: An electrically continuous rail that connects both power packs to complete the commonelectrical circuit. The other rail is called the “control rail” in which gaps are cut to define the electrical blocks.

Feeder: The wire connecting a segment of rail to the power supply.

Insulated rail joiner: A rail joiner made of an insulating material such as plastic instead of metal.

3 Join the discussionRead and share your comments on this article

Comment on this article

MICHAEL PENTECOST from

Where can I find a wiring diagram for a reverse loop using twocab control? Figure 2 shows cab control, and anotherfigure shows the reverse loop, but how do I combine them? Can one (or both) of the polaritychanging toggles becombined with the cabselector toggle?

Add a comment

Type your comment here...

If you are experiencing difficulties with our website or video player, please contact Customer Service

2/10/2015 How to wire a layout for twotrain operation | ModelRailroader.com

http://mrr.trains.com/howto/dccelectrical/2014/06/howtowirealayoutfortwotrainoperation 6/6

Added 8 months ago

Added 8 months ago

RICHARD J REITZ from NEW YORK

Hi nice article with clear explanations. Just one comment are the A and B designations on Figure 2 reversed? Seemslike the toggle switch in the "B" position would be controlled by Cab "A". Maybe I'm just confused ... thanks for the article.

Add a comment

DAVID NELSON from WISCONSIN

Thank you Neil for this refresher. There are still plenty of modelers using conventional DC (and indeed a case can bemade that this might be the time for the fencesitters to wait out the refinements of DCC in anticipation of the next advancewhich is certain to come sooner than you'd think: battery powered, radio controlled).

What is needed is a new edition of Andy Sperandeo's Easy Wiring book (which I am surprised has been allowed to go outof print) tailored for people who are sticking with DC, for now, and who know what DCC is and what it can do. At this pointhis entire chapter on DCC could probably be replaced with a few paragraphs. The suppliers for DC and electronics ingeneral have either changed (rarely for the better) and there are some fringe suppliers who could be included.

Also it is evident from the Forums discussions that there remains a great deal of uncertainty and confusion about Pecoand its turnouts in particular, which Andy only touches on in his book.

Add a comment