Crossings and DCC
Summary: A Crossing is where two railroad tracks cross each other at the same level.
Also known as a Diamond Crossing when the angle is less than 90 degress because the rails make a diamond shape.
Variations are Single-Slip and Double-Slip crossings that allow the train to pass from one track to the other. The single-slip version only allows this "slip" on one side whereas the double-slip allows it on both. Slips are a lot more complicated because the are effectively a crossing and two turnouts all combined into one.
Another variation is the Scissor Crossing where two opposing crossovers overlap each other. This compact arrangement allows trains to cross between two parallel tracks in either direction.
In general, any crossing that works with analog power will work with DCC. However there are a couple of issues peculiar to crossings that relate to DCC. The first is to do with locomotive pickups and the second is live frog switching.
Just like a turnout, a diamond crossing has frogs where the inner rails (the frog rails) meet at an acute angle but obviously in this case there are two frogs. A crossing also has guard (or check) rails just like a turnout to ensure that the wheelsets take the correct route through the frogs. The out rails are called the stock rails.
Slips have two sets of moveable switch rails, one set at each end. Each set has its own throwbar (or tiebar). Single-slips have 2 switch rails at each end whereas double-slips have 4. At least on model railroads, all four switch rails at each end of a double-slip share the same throwbar. While popular on model railroads, the prototype has been removing them for years, as they require a lot of maintenance. They were popular in tight quarters, such as a station with multiple platforms receiving multiple trains in short periods of time. It allowed for multiple routes in a compact space, compared to using turnouts and crossovers.
Single-slips appear to have 2 useful settings; both sets of switches "closed" (behaves just like a crossing) or both switches "thrown" (slip route is selected). However, as we shall see below, the other settings can be useful too.
Double-slips have 4 useful settings; slip one side, slip the other side, cross one way or cross the other way.
These settings are shown in the table below. Since double-slips are completely symmetrical, "thrown" and "closed" don't have much meaning but the table is written so as to be consistent with the single-slip version.
|Left switches||Right switches||Single-Slip||Double-Slip|
At first glance you may think that the thrown/closed and closed/thrown cases don't make much sense for single-slips since one straight route is fouled. If both are closed, neither straight route is fouled. However these cases can be used to our advantage when considering how to wire up live frogs.
A scissor crossing (or scissor crossover) is equivalent to 4 turnouts and a diamond crossing. There are some ready made ones available such as the Peco N gauge SL-E383F.
Sometimes known as a double crossover but that can also refer to two non-overlapping crossovers, one after the other.
See Wiring Crossovers for more details and a video
Generally, ordinary crossings (not slips) route power through the straight routes using factory installed bonding wires on the underside. There is no electrical connection between the two routes. This allows the two routes to be in separate DCC power districts. Also it doesn't matter if a reversing loop is involved (imagine the crossing at the center of a figure of eight), there will be no short circuit.
The diagram below shows a Peco Insulfrog type crossing with the rails in 4 colours to represent the different polarities and potentially different power districts.
Live frog crossings and slips are different, there will be an electrical connection between the two routes and therefore they should probably both be in the same DCC power district. See Wiring Crossings for more information.
- Main article: Turnout Operation
On a DCC layout, crossings can be operated remotely using Turnout Motors controlled by Stationary Decoders using commands sent from a Throttle. You need a minimum of 2 turnout motors for a crossing and some may need 4. This addional complexity is a good reason to consider automatic routing where setting a route will automatically throw all the turnout motors required to complete that route.
Model railroad crossings have two types of frogs; dead frogs and live frogs.
A dead frog crossing has insulated plastic or isolated metal frogs (and joints between the stock rails).
Dead frogs are a more serious problem with crossings than they are with turnouts because instead of just one dead section there are four dead sections. This greatly increases the chances that a short locomotive or one without many pickups will stall on the crossing.
The diagram below shows an 0-6-0 locomotive passing over a crossing where only 2 out of the 6 wheels are live. If either one of those two wheels doesn't have a pickup then the locomotive is dead. The other route is shown unpowered for clarity.
There are two approaches to solving this:
- Consider using live frog crossings instead (see below).
- Consider fitting Energy Storage Modules to your short locomotives to provide power while traversing the dead spots.
A live frog crossing has metal frogs to reduce the dead sections where there is no electrical pickup.
Live frogs present a similar problem as they do for turnouts; the polarity must be correct for the route in use. But with crossings there are two additional issues:
- Crossings have two frogs and both must be switched separately because they must always be the opposite polarity to each other.
- With ordinary crossings (not slips) there is no mechanical clue as to what the polarity should be, i.e. there is no throwbar to which an electrical changeover switch can be connected. So the polarity will always be wrong for one of the routes.
There are many solutions for these problems and they are discussed in Wiring Crossings.
The diagram belows shows a Peco Electrofrog type crossing. The blue and magenta frogs and frog rails must be polarity switched to match the desired route. Insulated rail joiners must be used on the frog rails.
Note that some live frog crossings do not have the insulating joint where the stock rails meet and therefore both routes must be in the same power district unless you gap the rails.