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This is a brief introduction to the (very technical) Automatic Train Control System.
What do I actually do for WMATA?
Jump to the non-technical explanation
The ATC system that provides automatic control of the trains on the WMATA rail transit system (METRO) consists of three subsystems and a computerized Central Control facility. The three subsystems are Automatic Train protection (ATP), Automatic Train Supervision (ATS), and Automatic Train Operations (ATO). Each subsystem performs it's own particular functions independently, to a certain extent, of the other two. The operations of the three systems are coordinated through the computer at Central Control to achieve an integrated real time control system.
The Central Control facility serves primarily to monitor the total WMATA rail transit system and to initiate speed command reduction corrections as necessary to provide a smooth traffic flow. Some correction commands are initiated automatically by the computer but correction strategies with potentially more serious disruptive effects to service are alarmed by the computer and require correction initiated by an operator.
The ATC system includes a Data transmission System (DTS) and a Cable Transmission System (CTS). The DTS transfers commands and indications between wayside facilities (train control rooms), (TCRs) and central Control, via the CTS. The CTS provides voice band transmission channels for the DTS and to support other communication needs.
Automatic Train Protection
The ATP assists in enforcement of safe operation of the system. it imposes speed limits both to maintain safe train separation and to operate trains in accordance with civil speed restrictions. At interlockings, (locations containing a network of track crossings), ATP ensures that train movement is permitted only when a clear, un-contested route is available through the interlocking, and the track switches are locked in position. In all cases where two or more trains are competing for the use of a common segment of track, the system allocates the track to one train at a time in an orderly fashion and locks out all others.

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Train Detection
Each WMATA mainline route is divided into blocks from the terminal station at one end, to the terminal station at the other end. Each block is checked for train occupancy by means of audio frequency (AF) track circuits. Except in interlockings, there are no insulated rail joints used for electrical block separation. Instead, block separation in interlockings is obtained by tuned impedance bonds, which also serve to inject into the track coded AF automatic speed limit commands. These commands are picked by the train. Bonds are located at each end of each track circuit. Generally, there is one track circuit per block. Each impedance bond (which serves as the track circuit boundary), is used as the transmitter coupling for one track circuit and the receiver coupling for the adjacent downstream track circuit. An ATP speed command transmitter at the leaving end of the track circuit feeds AF energy to both rails of a track, using the impedance bond as a coupling transformer. An impedance bond at the entering end of the track circuit act as a receiver energizes a track relay if the signal from the transmitter is not shunted by axles of a train. The transmitter and receiver associated with one track circuit are tuned to the same frequency. Adjoining track circuits use different frequencies to prevent the block (train present) from being energized if either adjacent block is unoccupied. Two track circuits of the same frequency have sufficient distance between them to allow the signals to become attenuated. This prohibits one track circuit from energizing the other's track relay.

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Train detection with the crossovers is accomplished by using an AC power frequency track circuit. insulated joints are installed on each crossover rail to isolate the AC track circuit from the AF track circuits. An AC signal of 120volts, 60HZ is applied to a track transformer and to the local coil of the AC van relay. The combination of voltages at the local and track coils of the AC vane relay energize the relay. When a train is in the track circuit, the ac track current is hunted and no voltage is present at the track coil. The ac vane relay de-energizes to indicate a train in the crossover. AC track circuits are used only in the crossover track circuit; all other circuits utilize AF track circuits.

A even shorter version that I tell people is that the trains run semi-automatically in ideal outdoor weather conditions. All the operator does is close the doors (for safety purposes) and the train - accelerates, maintain distance between trains, de-accelerates, pulls into a station at a precise location, stops, opens the doors and waits for the operator to close the doors again. Routing through the system is controlled in the Train Control Rooms (for example the divergence of the blue and yellow line at Pentagon Station) when the train sends a signal to the TCR indicating the type or run of the train. Central Control can over ride all aspects of the Train Control but vital safety aspects of the Automatic Train Protection system can not be controlled by central (all absolute safety functions are located in isolated and self sufficient TCRs). If weather is less than ideal (rain, snow, ice, unforeseen events) the operator is trained to take control of the train and take necessary action. The basic philosophy of Automatic Train Control is that all systems check for positive indications. Any negative indications ( for example: trains too close together, wayside intrusion) the system will slow trains to a stop with the operator making a train movement decision concurrent with Central Control's instructions.

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Where do I as a mechanic (ATC Repairer) fit into this picture? I repair the switches, electro-mechanical functions (relays) and electronic elements of the Automatic Train Control function (ATP high frequency modules, Remote terminal units, signal heads, Computer controls (Microlock, GRS-GL1) and snow melting equipment ).
I also assist Central Control in periods of equipment failures, adverse weather conditions and heavy passenger situations (parades, July 4th, etc.) Activities include routing trains through local interlockings, cranking switches to proper positions for routes. Also included in this description are routine inspections of above and below ground ATC equipment.

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