Conventions

• When a loco is mentioned, it is assumed that it is a loco with a DCC decoder installed and running on a DCC-controlled layout, unless otherwise stated. Examples are an analog loco for a loco without a DCC decoder, or analog layout for a conventionally block-controlled layout.
  
• "DPDT" means double-pole, double-throw. Double-pole means that it has two independent circuits - one for each rail. Double-throw means that it has two positions of on - as opposed to one position for on, and one for off. A typical DPDT device has six connections for the circuitry to pass through - three for each pole, with each pole having a common (output), and two inputs. More about wiring these later.

First, to dispel some mis-thinking of many people.

With DCC, the polarity on the track has nothing to do with the direction of a loco. You can reverse the polarity on the rail while a loco is moving, and the loco won't even flinch. So, if you choose to control a reverse section manually, you can simply use a DPDT toggle switch to control the polarity of the reverse section. You don't need to ever reverse the polarity of the rest of the layout like you have to with conventional control.

That said, let's get on with it.

There are basically four different ways to control polarity automatically - depending upon your track plan, ingenuity, attitudes, and budget. These are, in order of cost: slide switch connected to a turnout's linkage, relay connected to a turnout's electrical control system, Loy's Toys Automatic Reverse Section Controller (ARSC), and various manufacturers' auto-reversing boosters. This does not take turntable polarity into consideration, which will be discussed at length at the end of this piece.

There are special situations, conditions, and other circumstances that will determine which of these will be best for you and your situation. First, we'll discuss the conditions that must exist to be able to use the least expensive methods - slide switch and relay. Then we'll get into everything there is to know about the other methods.

If you have a "clean" reverse loop or a turn-around wye that has at least one dead-end leg, you can use either a DPDT slide switch or relay. A "clean" reverse loop is one that has only one turnout where the train can go in and out. This doesn't mean that the loop can't have other turnouts within it, to have yards, industry, or other sidings. It just means that there can't be any other way out of the loop to get back to the mainline. For details, refer to the Wiring Reverse Section Control application note.

If you use manual hand throws on your turnouts, use a DPDT slide switch connected to the turnout linkage as illustrated here.

Also, auto-reversing boosters have only two wires to connect to the reverse section (plus power into the booster), and Loy's Toys ARSC has six (plus power from the track bus) - two for the reverse section, and two more for each detection section.

However, there are other differences:

When using auto-reversing boosters, the reverse section must be longer than the longest train that has power pickup trucks, such as lighted passenger cars, cabooses with power pickup for lighting, freight cars with power pickup for FREDs, sound, or other electrical needs.

First, there are two manufacturers that make auto-reversing boosters: Digitrax and Wangrow. Digitrax boosters are designed to be triggered when there is any short circuit detected at all - even if it's only one gap that has been shorted. Wangrow's boosters require both gaps to be crossed at the same time for it to work reliably. This means some steam locos won't trigger it reliably, because they only pickup power from the right rail, and get left rail power from the tender.

If using Wangrow boosters for auto reversing, you need to either have all-wheel-power-pickup locos, or install power pickup to the left wheels of the locos that don't have all-wheel power pickup. Some people have tried to use Digitrax booster for auto reversing with Wangrow's system, but it won't work unless you have at least two Digitrax boosters.

For a steam loco to trigger a Digitrax booster, the loco must be leaving a section of track that is also powered by a Digitrax booster. If you opt for this, you can use one Digitrax booster (set to not auto reverse) to power the entry and exit of all your reverse sections, then use another Digitrax booster to power all the reverse sections.

However, if you plan to be able to use more than one reverse section at a time, you could cause a short circuit. To avoid this, you must have one booster for each reverse section. However, the one booster (set to not auto reverse) for all the entries and exits will work just fine.

Like Wangrow's booster, Loy's ARSC requires that the gaps of both rails be crossed by power pickup wheels at the same time. It works just fine with all-wheel power pickup of diesel locos, but won't work with steam locos that do not have all-wheel power pickup. The only advantage here is that the reverse section doesn't have to be longer than the longest train.

The ARSC was designed this way so it would know the difference between a loco and power-pickup car - so it will trigger when a loco crosses the gap, but not a power-pickup car. This was done specifically so reverse sections could be as short as possible, for the reasons previously discussed. Were it not for that, the ARSC could have been designed to be triggered by one gap only, but then the reverse section would have to be long, like with auto-reversing boosters. Besides, adding proper power pickup to your steam locos will simply make them run better.

The ARSC can control all of your reverse sections the same way an auto-reversing booster can. But, it has the same limitation too. If you have one train crossing a north polarity end while another train crosses the south polarity end of a different reverse section, you will have a dead short. However, the ARSC is low enough in cost that you can easily afford one for each reverse section.

• "Clean" loops and dead-end legs of reverse wyes can be controlled by a DPDT slide switch or a relay.
• Wiring of each type of control: Loy's ARSC is more complex than an auto-reversing booster.
• Reverse sections (except for "clean" loops and a wye's dead-end leg) should be kept as short as possible to lessen the possibility of crossing opposite polarities at the same time by two different trains.
• Length of reverse sections: Auto-reversing booster requires longer than train, Loy's ARSC requires only longer than MU lashup.
• Gap crossing: Loy's ARSC and Wangrow's auto-reversing booster requires both gaps be shorted, Digitrax's auto-reversing booster will trigger with just one gap being shorted.
• Cost: Manual control and Slide switch or relay connected to turnout control is the least expensive, Loy's ARSC is $29.95, Digitrax booster is $179.95, and Wangrow's dual booster is $249.50.
  
  Now for some hints and tips about reverse sections. These tips have been derived from analyzing many track plans for many customers to figure out how to handle their reverse section problems.

• Many people have the urge to wire double mainlines with opposite polarity (one track East bound, the other West bound, as illustrated at right) because that's what they're used to when wiring block control.

• But, if you wire them identically, as illustrated at right, you will eliminate any polarity problem at crossovers. If wiring them this way causes a polarity problem elsewhere, it will be easier to take care of it there than at the crossover.

• If you have two tracks coming together, where the two tracks are of opposite polarities, you can install the reverse section in the merging track.

• Or, if you don't have room to do that, you can install it on the mainline.