Railroad signal system interlockings or control points are periodically tested to ensure that they are fully functional. An interlocking is a collection of electrical and electronic assemblies including but not limited to relays, logic controllers, signal lamps, switch motors, timers, coding and decoding units, modems, and other miscellaneous components and connections. The purpose of the interlocking is to control and monitor a railroad control point such as an end or siding, crossover, or double crossover.
An end of siding is a single switch that branches a single track into two tracks. A crossover is a pair of switches that allows a train operating on a first track to cross to a second parallel track, but only when the train is operating in a specific direction. A train operating in the opposite direction on the second track may similarly cross to the first track when the switches are properly aligned. A double crossover consists of two single crossovers in sequence or, where space does not permit, overlapping one another. The double crossover allows trains operating in either direction on either track to cross to the other parallel track.
Many of the interlockings in the United States consist of one of these three types. However, in areas where additional tracks are involved or multiple routes intersect, more complex interlockings are implemented as required by the track configurations and the routing requirements.
An interlocking is controlled by a dispatcher and by the signaling equipment installed in bungalows or field cases alongside the tracks near the site. Monitoring signals sent to the dispatcher indicate any tracks occupied by trains, any signals cleared to a permissive state, and switch positions. The dispatcher can issue commands to clear a signal (enable or allow a permissive state to be displayed), restore a signal (disable or force a restrictive state to be displayed), or move a switch. However, the interlocking will respond to the command only if the conditions that exist at the interlocking are safe and thus allow the command to be safely implemented.
A significant purpose for the interlocking plant is safety. Some unsafe conditions that are prevented by the design and implementation of the interlocking plant are throwing a switch while a train is passing over the switch (causes a derailment), switching a train onto another track occupied by another train (causes a head on or rear end collision), or throwing a switch in front of a moving train that had previously been cleared to proceed and is now unable to stop in time when the signal or switch position becomes visible to the engineer. In addition, other unsafe conditions are monitored and can cause signal aspects to change to less permissive or even restrictive indications when they are detected. Examples of these are hand switch positions for sidings and slide fences. A hand switch that is manually thrown, usually to permit a switching engine to operate on the track, presents an obstruction or derailment possibility, and changes any approach signal to a restrictive state. Similarly, a slide fence detects possible track obstructions such as falling rock or landslides when the slide fence is breached, and changes the approach signals to a restrictive state.
An interlocking is protected by signals at the entry points. These signals are called home signals. In addition to preventing unsafe commands by the dispatcher, the interlocking also controls the home signal aspects displayed on the signals. The aspects displayed represent rules for proceeding that are well known and understood by the engineer operating the train. The types of information conveyed by these rules include allowable speed, position of the switch being approached, expected condition of the following signal, expectation of trains ahead on the same track either stopped or proceeding in the same direction, or expectation of other possible track obstructions. In these latter two conditions, the train may be allowed to proceed but is restricted to a speed that permits the engineer to stop within his visibility distance ahead on the track.
Signals may also display aspects that indicate the condition of the next signal down the track. Communications are sent between interlockings on pole lines or via coded signals transmitted in the rails. This allows one interlocking to communicate its state to adjacent interlockings and permits higher speed operation through several interlockings in sequence when the track has been cleared and the route safely lined through each interlocking. Signals at a given interlocking thus display aspects that may depend upon dispatcher commands received, conditions within the interlocking, and conditions at a following interlocking.
The interlocking is important to the basic safety of the railroad signaling system. Interlockings employ vital circuits designed and implemented to provide failsafe operation in a highly reliable manner. The circuitry typically uses gravity relays in vital circuits and is connected with heavy gauge wire protected by high quality, low leakage insulation. Lightning arrestors, crimped ring terminals, and stud-mounted connections are all employed to ensure high reliability. However, to ensure that the system is fully functional, periodic testing is mandated by the Federal Railway Administration (FRA) to ensure that the safety features remain effective. An operational test is performed every four years on every interlocking. Since there are many thousands of these interlocking plants situated along the railroads, considerable time, labor, and cost are dedicated to meeting these testing requirements.
Currently, testing is performed by a maintenance crew. In order to test an interlocking, the crew obtains track time from the dispatcher. This means that the dispatcher has given up control of the interlocking for the duration of the tests. Typically, the dispatcher gives up control of the interlocking under test as well as the adjacent interlockings, since control signals are generated by the adjacent interlockings to completely test the operation of the interlocking under test. Thus, train operation is suspended in this area for the duration of the tests. The consequence of this interruption in service is unwanted train delays and possible loss of revenue.
In order to test an interlocking, the maintenance crew operates the interlocking in all combinations and attempts to override the safety mechanisms by locally commanding unsafe conditions. These tests are broken down into a series of tests called Route Locking, Time Locking, and Switch and Signal Indication.
Briefly, Route Locking tests to ensure that a switch cannot be moved or an opposing route lined (enabled) once a home signal has been cleared to allow a train to pass. Restoring the home signal to STOP starts a timer that locks the interlocking and prevents any routes through the interlocking from being cleared until time has expired.
Time Locking tests to ensure that a switch cannot be moved or an opposing route lined (enabled) once a home signal has been cleared to allow a train to pass and a train has entered the interlocking. Detection of a train passing through the interlocking by successively shunting the interlocking track circuit and the following track circuit prevents the timer from starting. However, continued presence of the (long) train in the interlocking prevents the signals from being cleared or the switch from being moved until the train has passed completely through the interlocking. The signal may then be cleared again for a following train. The permissive aspect displayed, however, will be a function of the communication signals arriving from the following interlocking. If the train still occupies the block(s) between the interlockings, then the signal may be restrictive.
Finally, Switch and Signal Indication tests that all the indications reported to the dispatcher and the interlocking plant from the switch position monitors are operating properly, and that the signals display the correct aspect indications for all operating and communication input conditions.
These tests are complex and exhaustive. A number of maintenance workers are required. The realities of railroad operation may not allow sufficiently long blocks of track time to fully test an interlocking without releasing track time and allowing a train to pass through an operational interlocking. Communication among the maintenance workers on the test team is via telephone, portable radios, and shouting as required. This presents an opportunity for misunderstood commands and requests, erroneous reporting of results, and the need to repeat commands and steps until the test has been correctly performed.