Prototype Information: Interlockings

Interlockings are an important part of railroad operations.  They control areas where tracks come together.  They are also interesting areas to model, because they give the impression that the railroad is not limited to what is modeled.  Interlockings vary widely in size.  They can be as small as a siding switch, or large and extensive, such as the approaches to New York's Pennsylvania Station.

Interlockings get their names from what they do.  An interlocking is a system of switches and signal apparatus interlocked together in such a way that they must be operated in a certain order, and they prevent the operator from allowing conflicting movements through the interlocking.

"Armstrong" levers at Wilson Tower, on the Chicago Elevated.  Image
from www.chicago-l.org.

Many years ago, before electronics were used to control many railroad movements, interlockings were controlled by a person called the control operator.  He worked in an interlocking tower, from which point he could see most of the interlocking and any approaching trains.  In those towers, there was a series of levers, which directly controlled switch positions and signal aspects.  He would move the levers to line trains on a specific route through the interlocking.  In the bottom of the interlocking tower, there was a piece of equipment called a locking bed.  The locking bed is what made the interlocking work properly.  When the control operator moved a lever, it moved a rod in the locking bed.  The movement of that rod would lock out any levers whose movement would create a conflict.  For example, if the control operator moved a lever to give a train a proceed signal, the movement of that lever would lock out the levers for opposing signals.  This made it impossible for the control operator to create a situation in which trains would be able to run into each other.

A locking bed.  Levers were connected to the vertical
parts in the locking bed.  The notches pushed "dogs"
horizontally to lock out other levers and appliances.  Image
from www.wikipedia.org.

Each piece of the interlocking had a normal position.  For signals, the normal position was to show a stop indication.  For switches, the normal position was defined in the timetable, but it was always for the main track, and usually the through route.  This was not always the case, as it did depend on traffic levels, and which railroad controlled the interlocking when more than one railroad was involved.  When no trains were in the interlocking, or cleared through it, all the appliances would be in their normal position.  When appliances were reversed, or changed from their normal position, they would lock out other appliances and prevent any conflicting movement.

In order to make the interlocking to work properly, the control operator had to operate the various appliances in the proper order.  The first thing he would line would be switches.  Once the switches were lined, he would operate the switch locks, which not only locked the actual switch points, but also locked out the switch lever.  Next he would clear the appropriate signal into the interlocking.  Doing so would lock out any opposing of conflicting signals.  In some cases, the last thing he would do is clear the appropriate distant signal.  All of these appliances were interlocked in such a way that if the control operator lined a switch, he then could not clear a signal against that switch.  Likewise, once a signal was cleared, he could not clear any signal that would conflict.  Once a signal was cleared, he could not clear an opposing distant signal, and he could not line the switch.  Once a distant signal was cleared, the signal protecting the interlocking could not be changed.  While interlockings still exist today, most of them are controlled electronically.  Rather than move levers, the control operator changes something on a computer screen.  A set of physically interlocked levers and controls are gone too.  The computer interlocks all the interlocking apparatus electronically.

Now, the big question is how do you go about incorporating this into a model railroad.  The first thing you will need to do is design the interlocking.  The physical track plan is the first step.  You will need to decide where each switch and signal will go.  Once that is complete, you need to look over the track plan and figure out what all the opposing and conflicting routes are.  Some are easy.  If a signal is cleared for one direction, you cannot have one cleared for the opposite direction on the same track.  Others are more complicated.  You have to figure out every possible scenario for the interlocking, and in each scenario, you need to figure out every appliance that must be locked out.  This is usually the most time consuming part of the process.  Next, you have to decide if it will be an electronically controlled interlocking or an old fashioned, physically locking type.  For an electronic interlocking, you will want to program all the interlocks.  Exactly  how to do this goes beyond the depth of this post, but it is done through a series of if... then... statements.  If you choose to go the old fashioned way, you will need to make a locking bed, and then connect it to some sort of control levers (Some are available from www.humpyard.com).  Again, this goes beyond the depth of this post, but it is something we plan on talking more about in the future.

Not all interlockings are controlled.  Manual interlockings are the only type that have a person operating them (An easy way to remember this is a man controls a manual interlocking.).  Automatic interlockings are activated by the train crew, and then they operate automatically (For this remember that at an automatic interlocking, the train crew automatically gets off their butt and does something.).  This is often found where two railroads cross, but neither one is particularly busy.  At the crossing, there will be a box that houses the interlocking apparatus, and mounted on the outside will be a pair of buttons.  When a train approaches the interlocking, it stops, the Conductor gets out, and pushes the button for the intended route.  The interlocking apparatus will take a minute to search for any other trains, and if none are found, and no conflicting route is found, it will display a proceed signal indication for the crew that pushed the button.  In some places, trains enter and exit sidings this way.  They stop short of the switch, the Conductor pushes a button  mounted on the signal mast, and then the switch lines and the signal indicates proceed.  At automatic interlockings, if another train is already present within the limits of the interlocking when the button is pushed, it will wait until the intended route is clear before it tries to line any switches or change any signals.

A gate protecting an interlocking.  Photo from forums
at www.trains.com.

Some interlockings are even less complicated than that.  In areas of very low rail traffic, sometimes the only thing controlling an interlocking is a gate or stop sign.  Typically these are only used where lightly traveled rail lines cross.  In the case of a gate, one route will normally have the gate across it.  When a train approaches on the other route, they may proceed without stopping.  If a train approaches on the gated route, they must stop, and move the gate so that is blocks the other route.  The train then proceeds through the interlocking, and once the entire train is clear, the gate is put back in its normal position.  Some crossings are simply a four way stop.  There is a stop sign on each track approaching the interlocking.  When a train arrives at the interlocking, it stops, the crew looks both ways, and if there is no opposing traffic, they proceed.  In the event that more than one train arrives at the stop sign at one time, the crews communicate with each other to decide who will go first.