Exponent has the experience and capabilities to investigate and asses the conditions of the signal systems and its component to assist in establishing programs for the ongoing maintenance, repairs, and modernization of signal systems for rapid transit, medium and light rail systems.
A signaling system provides each train with the authority to occupy the main track before it can begin moving. There are several types of authorities, but usually only one type is in effect on any given piece of track. The common types of authority used to direct train movements today are interlocking signals, Centralized Traffic Control (CTC), Rule 251, Track Warrant Control (TWC), Direct Traffic Control (DTC), Form D Control System, and Yard Limits.
A look at the different types of authority under which trains may move:
Interlocking Signals. Interlocking signals permit movement within the limits of the interlocking plant bounded by those signals. As their name implies, interlockings are arrangements of signal appliances (once mechanical, now electronic) that are designed so that conflicting movements cannot be authorized. Interlockings may be manually or automatically controlled. Automatic interlockings are most common at the remote crossings of two railroads, and operate on a first-come, first-served basis.
Centralized Traffic Control. CTC, sometimes called TCS (Traffic Control System), is familiar to train watchers. CTC is commonly found on high- or medium-density lines, where the signal equipment cost can be justified by the reduction in train delays. Conceptually, CTC is a series of interlockings all controlled by one person. Trains are governed by signal indications, some of which provide movement authority. The traffic-control machine or computer software is designed so that conflicting authorities cannot be granted. From a console, now typically a computer screen, the dispatcher remotely controls signals and powered switches, which are most often found at the ends of sidings and at crossovers between main tracks. Trains need only to observe the controlled signals to obtain movement authority.
Rule 251. Rule 251 allows a train to operate on signal indication, but only with the current of traffic established for the track. Rule 251 is usually used with Double Track. ("Double Track" is not defined the same as "Two Main Tracks" on Western roads. Double Track has a designated current of traffic for each track, and is signaled only for trains moving in that direction. Two or more Main Tracks are each signaled for movement in either direction, and controlled by CTC.) The signals in Rule 251 territory are controlled by track occupancy, not the dispatcher. This type of operation is common on commuter lines where most or all trains operate at the same speed.
Track Warrant Control. Track Warrant Control authorizes the dispatcher to verbally instruct the train to proceed, usually via radio. The dispatcher selects the stations or mileposts between which the train may move - a segment of track known as the authority limit. Warrants may contain conditions such as "not in effect until after the arrival of ..." The train crew writes the instructions on a Track Warrant Form and repeats them to the dispatcher for verification. Nothing prevents the dispatcher from erroneously issuing overlapping or conflicting warrants, though most large railroads now require him to simultaneously key the authority into a computer, which checks for conflicts. TWC is used by most Western roads and Norfolk Southern on medium- and low-density lines. The TWC Form is also used in CTC territory but not to authorize movement. It serves as a checklist of Track Bulletins the train crew should have received, like the old Clearance Form A used with train orders. (Track Bulletins address maintenance work and track conditions, including temporary speed restrictions.)
Direct Traffic Control. Direct Traffic Control is similar in execution and application to TWC, but the railroad is divided into pre-defined "blocks." The dispatcher authorizes a train to proceed in one or more of the blocks but does not have the flexibility in the selection of authority boundaries available under TWC. Kansas City Southern, Union Pacific, and CSX are three railroads that use DTC on portions of their routes.
Form D Control. A Form D Control System (DCS) is used by Northeastern railroads that have adopted the NORAC Rule Book. It is a variation of TWC. Canadian railroads, whose dispatchers are called Rail Traffic Controllers, use an Occupancy Control System (OCS) Clearance also similar to TWC.
Yard Limits. Yard Limits authorize any train to move at a speed that allows it to avoid conflicts. This is the railroad equivalent of aviation's visual flight rules. A yardmaster may direct yard movements, but does not provide movement authority. Trains and engines must still watch out for each other. Yard limits can apply beyond yards, such as on branch lines where speeds are slow and train meets are rare.
TWC, DTC, DCS and Yard Limits do not require wayside signals. When signals do exist, they serve primarily as a protective overlay to the movement authority and do not convey the authority to occupy the main track.
Protecting track workers:
Each system of train movement authority must also include provision for protecting maintenance vehicles and workers, as well as any work trains that may be moving back and forth within a section of track.