Crossovers and Slips

A crossover allows a train to move from one track to another, as opposed to a crossing which allows one track to cross another.
:If you are looking for crossings, go to Wiring Crossings 
Many crossovers are a simple arrangement of turnouts which provide a path from one track to another. Another variation is the Scissor or Double Crossover where two opposing crossovers overlap each other. This compact arrangement allows trains to cross between two parallel tracks in either direction.
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.
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.
==Overview==
=== Slip Switches === 
==== What is the Difference between Single and Double Slip? ====
The difference between the two is rather simple.
# A Double Slip allows for two possible routes in either direction. 
# A Single Slip offers two possible routes for one track through the crossing. For the other track only one route is possible, with the single slip functioning as a crossover only.
Image:CrossingSingleSlip.gif‎|A Single Slip Showing Route Options. Note that it only has two sets of switch rails.
Image:CrossingDoubleSlip.gif‎|A Double Slip Showing Route Options]]
Left switches	Right switches	Single-Slip	Double-Slip
Closed 	Closed 	 	
Closed 	Thrown 	 	
Thrown 	Closed 	 	
Thrown 	Thrown 	 	
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). As the diagram shows, Route A has two routes available to it, where Route B has only the crossover route. 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. 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.
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.
=== Scissor Crossing, Single and Double Crossovers ===
thumb|500px|Double crossover.
* Single Crossover
** Trains can move from one track to another from a single direction. See image at the top of the page
* Double Crossover
** Trains can change to either track, from either direction
* Scissor Crossover
** Same as a double crossover, but requires much less real estate 
A scissor type 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. The scissor crossing allows a train from either direction to change tracks. This arrangement is much more compact than using a double crossovers for the same purpose.
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
== Frog Type Crossings ==
----
Model railroad frog type crossings have two types of frogs: Dead and Live frogs. 
Image:CrossingInsulfrog.png|Electrical View of a PECO Insulfrog Crossing with a dead frog
Image:Crossing060.png|0-6-0 Locomotive on a Frog type Crossing. Notice that 4 of the six wheels are on a dead spot.
Image:CrossingElectrofrog.png|Electrical View of a PECO Electrofrog Crossing with a live frog
=== Dead Frogs ===
A dead frog crossing has its from made of plastic or if made of metal, it is isolated from the switch and has gaps on its point rails. 
Dead frogs are more of a problem with crossings than they are with turnouts, instead of just one dead section there are four dead sections. This greatly increases the chances that a short wheelbase 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.
=== Live Frogs ===
A live frog crossing has conductive 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 phase must be correct for the route in use. With crossings there are two additional issues:
* Crossings have two frogs, both must be switched separately because they must always be the opposite phase to each other.
* With ordinary crossings (not slips) there is no mechanical clue as to what the phase should be, i.e. there is no throwbar to which an electrical changeover switch can be connected. So the phasing 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 below shows a PECO Electrofrog type crossing. The blue and magenta frogs and frog's point rails must be switched to match the phase of the desired route. Insulated rail joiners must be used on the frog's point rails.
Note that some live frog crossings do not have the insulating joint where the stock rails meet, therefore both routes must be in the same power district unless you gap or isolate the stock rails. Doing this eliminates phase issues between two districts.
=== Power Routing ===
==== Single and Double Slips ====
Slips are the slightly more straightforward case because the turnout motor that controls the switch rails can also be used to switch the frogs. The key thing to remember is that each frog should be fed from the opposite turnout motor accessory switch.
Single slips have one case (both sets of switches closed, not thrown) that causes a bit of a problem. In this case both routes are available and each route will have one frog phase wrong if the simple wiring described above is used. There are a couple of solutions to this:
* Use automatic turnout routing to prevent this case from happening. The 3 remaining cases cover all possible routes.
* Use a more elaborate circuit that combines the accessory switches of this single slip and a related turnout to infer the route in this case.
==== Dead Frog Slips ====
Dead frog slips can be wired up essentially the same as dead frog diamond crossings except that all routes should be in the same power district. 
If a reversing loop is involved (one end of the crossing loops back around on itself) then a DCC Auto Reverser will be required for the loop and insulating rail joiners or gapped rails must be used.
==== Live Frog Slips ====
Live frog slips can be wired up in a similar manner to live frog crossings as follows:
* Use a DCC Auto Reverser or two DCC Frog Juicers as described above for diamond crossings.
* For double-slips use the crossing's turnout motors' accessory switches to switch the frog polarities as described below (see #Single and Double Slips).
* Single-slips are a little trickier to wire using accessory switches because there is one setting of the switch rails which makes it equivalent to a diamond crossing (see #Single and Double Slips).
====Diamond and Scissor Crossings====
Scissor crossings can definitely use this method whereas diamond crossings can only use it if there is a related turnout that can be used to infer the selected route. For example, if the crossing is due to a junction on a double track main line and the turnout is set to the diverging route then the only safe route through the crossing is the diverging route. The main line that it crosses should be signalled to reflect this.
The diagram below shows an example junction where the down line to branch has to cross the up line. The crossing's frog polarities are set by the down line turnout. Note that this junction is simplified for clarity, in reality there would either be another turnout on the up line for trains returning from the branch line or the crossing would be a single-slip.
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 shows a Peco Insulfrog type crossing with the rails in 4 colours to represent the different phases 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.
=== Remote 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 additional 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.
====Turnout Motor Accessory Switches====
This section will assume that the mechanical method of controlling frog phasing uses one or two SPDT switches attached to or integrated into the turnout motor. There are other methods (such as manual throws with built-in switches) but these are effectively the same for the purposes of this discussion.
This solution has to be divided into two categories; slips (which have switch rails) and diamond crossings or scissor crossings (which don't).
File:DoubleSlipFrogs.gif|A Live Frog Double Slip Crossing with Turnout Motor Frog Switching
File:CrossingJunction.gif|Diamonds and Crossovers: A Crossing at a Double Track Junction
=== Double Crossover Wiring Diagram ===
;This image was used with the kind permission of Fast Tracks.
== Videos ==
=== Single and Double Slips ===
=== Wiring an Electrofrog Crossover ===
== Wiring a Double Crossover for DCC ==
The double crossover is a cool piece of track, due to its complexity some model railroaders may shy away from it. In reality, a double crossover is four turnouts and a crossing, which are not any more difficult to wire than ordinary turnouts and crossings. 
The video shows an N scale code 55 double crossover from Peco, which is nicely isolated out of the box, but the theory applies to all double crossovers.
==Further Reading==
* Crossings
* Wiring Turnouts
* Turnout Motors
=== References ===
* Wiring Point-work & Special track conditions for DC or DCC from DCC Concepts Ltd.
Category:Wiring
Category:Trackwork