In the first section of this overview we look at main track authority that relied on the superiority of trains to manage the movement. However there are systems that supersede the superiority of trains, they use some other method (typically signal indication) to control movement. Those methods are current of traffic, interlockings and CTC.
This will be an overview of those main track authority methods. It will not be exhaustive, just a general look at the those types of authority, what their key attributes are and some suggestions on modeling.
I will generally be discussing the rules as I was taught, which would be those applicable in the US. There are many different rules books and rules have changed over the decades in response to new technology, better communication and attempts to improve safety. Because different railroads have different histories, different physical plants, different geography and different managements, the details of rules can vary between eras and railroads. The rules and operations I will discuss are generally applicable from roughly World War 1 to the 1980’s. The basic concepts though, cut across all railroads and all eras. Some basic concepts in 1870 rules books are still in 2019 rule books.
As such, consult the rule book appropriate to your era and railroad for specific details. I’m sure that anybody can find a rule book and rule that will conflict in the details with some of the things in this article.
Superiority of Trains
Most people would call these systems “timetable and train order” because those are the ways that superiority of trains are communicated. It was basic, fundamental method of operating trains from the mid 1800’s up until the 1980’s in North America.
The definition of superiority of trains is :
Trains are superior by right, class or direction. Class and direction are conferred by timetable, right is conferred by train order. Right is superior to class and direction, direction is superior between trains of the same class.
Superseding Superiority
What that means is that the timetable schedule or a train order (“right”) doesn’t control or authorize the movement of the train. What that doesn’t mean is that the timetable doesn’t apply or that train orders aren’t used. They both still are there and the attributes of both not connected to authorizing train movement are all still in force. All the departure times of a regular train are still in force, all the speed restrictions, train operation restriction and special instructions in the timetable are still in force. Train orders not dealing with train movement or authority (speed restrictions, wide loads, tracks out of service, etc.) are all still used. Trains still need a clearance at all the places they would need a clearance in a TT&TO territory because trains still get train orders. The difference is that the trains get their authority to move from some other method other than a timetable or train order.
Current of Traffic
Often called “double track “ or “Rule 251”, it was the most common way to operate multiple main tracks from the 1800’s up through the 1960’s or 1970’s. As you might have guessed rule 251 in most rule books applied to this type of operation. On two main tracks, each track is operated in one direction and the other main track is operated in the other direction. Each track has signals for the current of traffic, but no signals in the other directions. The eastward track only has signals for eastward movements and the westward track only has signals for westward movements. The signals are typically an automatic block system and are not controlled by the dispatcher. Trains receive a clearance and then a proceed signal is their authority to move. Since movements against the current of traffic on the other main (operating westward on the eastward track) are actually being done in dark or unsignalled territory, they are authorized by train order. If the dispatcher wants a train to clear to let a train pass it, they will give the train they want to clear a message (rather than a train order).
Current of traffic is an awesome system for moving huge numbers of trains. As long as nobody stops. Then things back up and its cumbersome to pass trains on the other main track around the stopped train. That is probably why even though it was one of the most common systems used on real railroads, it is one of the least used systems on model railroads. Model railroads tend to have fewer situations where you want to pump many trains over a longer distance without stopping to do work. While it might not be good for an entire layout, it could certainly have applications where you have two main tracks and just want to always run trains in on direction on the same track, even for shorter distances.
Interlockings
An interlocking is typically defined as an arrangement of signals arranged to allow movements to succeed each other in proper sequence. What that means is it allows one move at a time over a route and any conflicting moves are stopped until the current move clears. In interlockings the signal indications also supersede the superiority of trains. The priority and sequence of movements is determined by the control operator, dispatcher or signal circuitry. There are two types of interlockings, manual and automatic. A manual interlocking has a “control operator” that manually sets the routes and clears signals. That can be anything from a simple diamond crossing with no switches up to a huge tower at a major passenger terminal that might have 4 or 5 operators working at the same time and hundreds of “levers” or controls for the switches and signals. At a manual interlocking the operator (who can be the dispatcher) decides which move goes first. Normally interlockings with controlled switches are manual interlockings.
An automatic interlocking is one where there is no control operator. The signal system decides which route gets the signal, normally determined by the order and timing when trains activate approach circuits to the interlocking. A dispatcher can’t control the signals at the interlocking and can’t clear a train through an automatic interlocking. If a train is stopped at an automatic interlocking and there is no conflicting move approaching , the train crew has to operate the circuit and follow a procedure written in the control box at the interlocking to get through it.
Normally the signals at an interlocking are placed at the entrances to the interlocking and are called “home” signals. If there are any signals at the exit of an interlocking (leaving signals) they are not part of the interlocking itself and may be controlled by the dispatcher. The limits of an interlocking are between opposing home signals. Interlocking signals may be tied into the block signal system (if there is one) and may also be block signals. If there isn’t a block signal system, the interlocking signals only apply to track within the limits of the interlocking. Trains are typically prohibited from partially occupying an interlocking and reversing directions (it confuses the signal circuits). A train has to fully occupy the limits before (passing both home signals) or completely clearing the interlocking before it can reverse directions.
Interlocking signals may be the same as block or CTC signals or they may be a special set of signals only used by interlockings. If you see a signal mast with three signal heads, its probably at an interlocking. The signal indication at an entrance typically govern the entire route through the interlocking. It is relatively rare to have intermediate signals inside an interlocking. Most railroads prefer not to do industrial switching in an interlocking if they can avoid it. Using hand operated switches is much faster.
CTC – Centralized traffic control
Centralized Traffic Control (CTC) is a system where trains move in both directions on a track where the signal indications supersede the superiority of trains. It can be on one or multiple tracks. It can also be known by different names or acronyms on different railroads. It is has gradually replaced much of the Rule 251 territory on multiple track over the 1960’s to current time frame.
CTC has points where the dispatcher or control operator controls the position of switches and can clear routes through those switches or tracks. Those are called “control points”. Between the control points the main track is generally signaled in both directions with an automatic block system.
The dispatcher doesn’t actually control the signals per se (other than not clearing a route and causing them to display stop), he or she can’t pick which indication the signals give. The dispatcher lines switches for a route and then causes the signal system to display a signal for that route. Based on the position of the switches, the route selected and track occupancies, the signal system displays the most favorable indication possible. That could be any combination possible at that signal, from stop to clear.
The benefits of CTC are flexibility and visibility. With both CTC and interlockings, a dispatcher or operator doesn’t need to commit to a route until shortly before the train arrives. If circumstances change, it takes a short time to change things. The signal system will put the signals and route to stop and wait for several minutes to make sure any approaching train won’t be surprised, then will allow the dispatcher or operator to change the route. The dispatcher can see the approximate position of the train (which 10-20 mile long block the train is in) and since the movement authority is communicated by signal, there is no need for written or verbal communication.
The down side of CTC, from both a prototype and model perspective is cost and complexity. A full CTC system requires occupancy detection, remote control of control points, detection of switch position and a system to drive the different signal indications. Fortunately for the modeler, there are several options for off the shelf electronics or software and it no long has to be hand crafted. Many modelers use a “lite” version of CTC that doesn’t have occupancy, but just indicates switch position.
Other systems
There may be other systems unique to specific railroads in this category. Any time the phrase “that supersedes the superiority of trains” is used it falls into this group. For example the P&R used a “train on branch signal” whose indication superseded the superiority of trains on the branch. If was basically a manually operated semaphore that a crew set to stop when they entered a branch and restored to clear when the left the branch. When they set it to stop they could use the branch in either direction. Any other train that wanted to enter the branch and found the signal set at stop would have to proceed under flag protection.
Next we will look at systems that were developed for more modern eras.