Turnout

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Short Definition

Arrangement of rails to allow a train to be switched between the main route and a diverging route

Atlas HO Turnout

A Turnout is the arrangement of rails to allow a train to be switched between the main route and a diverging route. In model railroad terminology, turnout usually refers to a ready-made track item that can be incorporated into a layout, but the idea is the same.

Also known as a switch, set of points or simply points.

You do not have to buy turnouts labelled "DCC-ready", and you do not have to modify existing turnouts when converting a layout to DCC. You should choose the turnouts that best suit your overall requirements and don't go out of your way to find "DCC-ready" or "DCC Friendly Turnouts".

  1. All turnouts work with Digital Command Control
  2. There are no turnouts that do not work with Digital Command Control
  3. If a turnout works with analog it will work with Digital Command Control

Terminology

Many use the terms turnout and switch interchangeably.

In prototype practice:

A COMPLETE SWITCH consists of two switch points with all the necessary accessories.
A SWITCH is not a TURNOUT, only part of it. Branching from one track to another requires a turnout with both a frog and a switch.

A complete turnout consists of:

  1. A straight stock rail
  2. A straight closure rail
  3. Two switch rails
  4. A curved stock rail
  5. A curved closure rail
    1. The closure rails will form the wing rails in the frog
  6. The frog
  7. The point rails
    1. The point rails for the heel of the frog as the rails exit the turnout.

The frog consists of a number of parts.

The main parts are the LH and RH wing rails, which form the TOE of the frog. The HEEL of the frog is formed by the Long and Short point rails. The HEEL has the flangeways between the point and wing rails. The THROAT is the section where the wing rails diverge to the just before the point formed by the point rails.

HO Scale turnout with parts identified
Switch-parts.jpg

Frog Numbers

The number of a frog is the ratio of the spread to its distance from the theoretical point (where the long and short point rails would form a point, not the actual point).

A number 4 is 1" in 4". The spread is measured at right angles to the center line. The actual angle formed is 14 degrees 15 minutes.

As per prototype practice, for high speed movements a turnout of #16 to 20 is recommended. Mainline slow speed movements, #10 to 12. Yards and sidings, #8. For modelling purposes, #8 is good for most applications, with smaller frogs angles for yards and branch lines.

Frog number Radius, Feet Degree of Curve (DMS) O Scale HO Scale N Scale
5 177.8 32-39-56 44 24 13
6 258.57 22-17-58 65 35 19
8 487.28 11-46-44 122 67 37
10 779.39 7-21-24 195 107 58
12 1104.63 5-11-20 276 151 83
16 2007.12 2-51-18 502 275 151
18 2578.79 2-13-20 645 354 193
20 3289.29 1-44-32 822 451 247

O, HO, and N Scale are expressed in Inches.

As the table shows, a number 20 turnout requires a radius of 21 feet in N Scale. So for modelling purposes, a number 10 or 12 turnout is reasonable compromise at five to seven feet (in N).

Digital Command Control and Turnouts

To say the least, DCC has certain requirements that must be met by your track work. The two electrical issues are related are the switch (points) rails and the frog. Using your NMRA gauge to verify the clearances of the turnout as well as wheel gauge goes a long way here.

  • Short Circuits are a disruptive problem and should be minimized. Since full power is available on the rails at all times, a short with cause interruptions in your enjoyment. They can also allow enough current to flow to weld things together which is never a good thing. Your boosters will react quickly and shut down everything, and if the short isn't cleared, will begin cycling on and off until it is cleared.
  • Wheels on Rails is not the most reliable power delivery system. Your locomotives need a clean, uninterrupted DCC signal on the track for proper operation. Electrically unreliable turnouts will cause problems like stalling, or in the case of sound decoders, restarting of the prime mover.
  • A careless operator can foul a turnout, causing a short, causing the DCC signal to be cut off. Or it may just cause problems until the points are aligned correctly, inconveniencing everyone else until power is restored.
  • You want to run your trains and have fun, and troublesome turnouts are not going to improve the experience.

Older turnouts may present issues with DCC. When they were made DCC wasn't available or not widespread. Later versions may have been improved to deal with issues arising with DCC.

Overview

Also see the page on DCC Friendly Turnouts.
Component Parts of a Turnout

Whether we're talking about full-size turnouts or models, the terminology is pretty much the same. A turnout consists of the main stock rails on the outside of each route. Where the rails have to cross each other is called the frog and the rails leading into this are called the point rails (incorrectly identified as frog rails in the picture). The moveable parts of a turnout are called the switch rails (identified as closure rails) and these are held in position by a tie-bar which can be moved sideways to select one route or the other. The end of the switch rails are tapered to form points or point blades so that they fit tightly against the inside of the stock rails. Finally, there are check rails (or guard rails) which ensure that the wheels on the opposite end of the axles take the correct route through the frog.

This diagram shows a turnout in the two possible positions. The top of the diagram shows the turnout set to the main route whereas the bottom shows it set to the diverging route.

Remote Operation

Main article: Turnout Operation

On a DCC layout, turnouts can be operated remotely by Turnout Motors controlled by Stationary Decoders using commands sent from a Throttle.

Types of Turnouts

Commercial turnouts are produced in many different shapes, sizes, sleeper types, rail profiles, and degrees of realism. To complicate the choice there are various different electrical behaviours and some are labelled "DCC-Ready" or "DCC-Friendly" and others are not. So, which type should you choose?

You do not have to buy turnouts labelled "DCC-ready" and you do not have to modify existing turnouts when converting a layout to DCC. You should choose the turnouts that best suit your overall requirements and don't go out of your way to find "DCC-ready" or "DCC Friendly Turnouts". However there are several special considerations to bear in mind and if you're just starting out on a DCC project it will be well worth investing the time to understand the different types of turnouts, their various benefits and the best way to wire them up.

But before you read on and get too confused, remember a few rules...

  • All turnouts work with DCC
  • There are no turnouts that don't work with DCC
  • If a turnout works with analog it will work with DCC

Power Routing

Non-Power Routing Turnouts

Non-power routing turnouts are ones where all routes are always powered. In commercial turnouts, this is achieved with internal bonding wires (usually wires on the underside) between the stock rails and point rails or stock rails and closure rails (the diagram shows the latter case but they are effectively equivalent to each other). Since the stock and switch rail pair is electrically connected, there should be no shorts caused by a wheel bridging the gap between them.

Non-Power Routing Turnout


Atlas and Roco are examples of non-power routing turnouts.

Self-Isolating or Power Routing Turnouts

Self-Isolating Power Routing Turnout

Self-isolating power routing turnouts are ones that only power the track on the selected route, the other route is electrically isolated. This is a good feature for analog layouts because it automatically isolates a locomotive sitting idle in a siding. But not so good for DCC because the whole point of DCC is you don't need to cut the power to make a locomotive stop and doing so will prevent things working; like sound, lights, and CV programming. The solution is simple though, just add power feeds to the track in the siding.

Peco Insulfrog and Hornby are examples of self-isolating power routing turnouts.

Insulfrog showing power routing


Non-Isolating Power Routing Turnouts

Non-Isolating Power Routing Turnout

Non-isolating power routing turnouts are ones that only power the track on the selected route, the other route has the same phase on both rails. To a locomotive this looks like no power. Like the self-isolating trunouts this is not good for DCC. And whether you're using DCC or not, this type of turnout needs isolating rail joiners or gaps (not shown in the diagram) on the end of the frog rails to prevent short circuits when power is also fed from elsewhere.

These turnouts have all of their point rails, closure rails and points electrically connected together. In some cases this is partly because the closure rails share the same metal hinge mechanism. The frog may or may not be live, in the diagram below a live frog version is shown. Shorts may occur if a wheel bridges the gap between the switch and stock rail.


PECO Electrofrogs are examples of non-isolating power routing turnouts.

Shinohara Turnouts (Walthers) come in two varieties. The older ones are non-isolating power routing. They can be identified by the metal bars joining the switch rails together and there are no gaps at the toe and heel of the frog. Small metal tabs under the points connect the switch rails to either stock rail.

Frog Types

The frog is the part of the turnout where two rails cross each other.

Frogs can be live (powered metal) or dead (plastic or isolated metal).

Live Frogs

Live frogs are metal and are electrically connected to the track power. They have the advantage of less unpowered dead track but are slightly more complicated to wire up.

There are two ways that live frogs can get their power:

  • Self Powered
  • Frog Switching
Just because the frog is constructed from metal (a casting or with rails) does not mean it is live.

Dead Frog

The dead frog means it is isolated from the rest of the track. Typically the frog will be made from an insulating material such as plastic, but metal can be used as well. This route made for turnouts which were much more DCC compatible than older turnouts with live frogs. Also at less cost. The PECO Insulfrog is a good example, as are many other brands, such as the Atlas Snap Track and almost all turnouts made with brass rail. Also available are turnouts by Micro Engineering, Walthers, the Atlas Custom Line Code 83 turnouts and others with metal frogs insulated from the adjoining rails.

Self Powered

With the self-powered or power routing type of turnout, the frog is electrically connected to point blades and therefore takes the phase according to the route selected by way of the blade contacting the stock rail.

The main task to perform is to gap the two rails that meet the frog. With ready-made turnouts this simply means installing insulating rail joiners on those two rails. The second task you may want to perform is wiring a frog feed that also provides the correct phase to the frog. This avoids having to rely on the blade to stock rail contact for a good electrical connection.

Peco call these Electrofrogs. Peco's live frog turnouts are ready to use out-of-the-box (assuming you use two insulating rail joiners on the frog rails) as the self-powered type. However they are designed to be easily converted to the other type (frog switching) if desired.

Many brands offer these types of turnouts such as Micro Engineering's older turnouts, Shinohara, Walthers and PECO's Electrofrog.

Frog Switching

DCC Friendly Turnout with Frog Phase Switching

With frog switching, the frog is isolated from all the other rails and must get its power from a separate power feed that switches accordingly.

The next diagram shows "DCC-Friendly" turnouts with the frog phase switch added. Here a switch is mechanically linked to the tie-bar so that whatever operates the points also operates the frog switch. This can either be a turnout motor with a built in switch, or a SPDT switch mounted at the side of the layout with a rod or wire-in-tube linking it to the tie-bar. An alternative to a mechanical switch is an electronic device such as the Frog Juicer.



Insulated Frogs

Insulated frogs are made of plastic and therefore don't conduct track power. These turnouts are easier to wire up but obviously there is a dead spot that can cause a locomotive to stall, especially short locomotives with limited pickups.

Peco calls these Insulfrogs. Peco's insulated frog turnouts are ready to use out-of-the-box with the slight caveat described below (see Small Frogs).

Insulated frog turnouts are shown in the picture earlier for Self Isolating Turnouts.

Isolated Frogs

Isolated frogs are made from metal but are electrically isolated from all the other rails which makes them behave very much like insulated frogs. However, being metal, this gives you the option to convert them to live frog by attaching a feed wire to the frog.

Isolated frog turnouts are shown in the picture earlier for #Non-Power Routing Turnouts.

Short Circuits

Three types of short circuits can occur at turnouts. Most of the time none of these should be an issue but if you want to improve the reliability of your layout take note.

None of these issues are specific to DCC but since a short on a DCC layout trips the booster and cuts power to all of the locomotives in the same district, the effect is much more annoying.

Small Frogs

The smaller an insulated frog is, the less dead track there is to inadvertently stall your locomotive. However if they are too small and your rolling stock wheels are too wide there is a risk that a metal wheel will bridge the gap between the two rails where they meet at the frog. Peco Insulfrog turnouts can have this issue, especially with older, cruder profile wheels.

If this proves to be a problem for you, see the page on the Peco Insulfrog for a simple fix.

Wheelbridging.jpg
Peco Insulfrog Short. The wheel is bridging both point rails on the heel of the frog.


Stock Rail to Point Blade Shorts

Points-shorted.jpg
Peco Electrofrog Short caused by wheel bridging gap between the switch rail and stock rail.

This is another issue that can occur with crude profile metal wheels if they are wide enough such that the back of the wheel flange can touch the side of the open point blade. Insulated frog or live frog turnouts can have this issue depending on how the point blades are wired. Ideally they should be isolated from the frog and electrically connected to the adjacent stock rail.

This issue can occur with Peco Electrofrogs because out-of-the box both point blades are electrically connected to the live frog which means the open blade will have the opposite phase of the adjacent stock rail. Peco supply instructions with all their Electrofrog turnouts for modifying them to solve this problem.



Running into Turnouts the Wrong Way (Against the Points)

Wrong-Way Live Frog Short

Any type of setup (DCC-Friendly or not) with a live frog has the hazard of a potential short circuit when you run into a turnout set against you. By definition, the live frog is set to the correct phase for the correct route and therefore the wrong phase for the wrong route. If a metal wheel bridges the gap at the frog, or the locomotive's front and back pickups span the gap then there will be a short. If you don't want this to trip your booster or circuit breaker you can either use a Frog Juicer to automatically correct the phase, or a Frog Current Limiter to reduce the current that can flow through the shorted frog.



See Also

References

USS Track Work Catalog, First Edition, 1955