Layout
In DTC, controlled tracks and sidings (those requiring authority from the train dispatcher to occupy) are divided into pre-specified blocks. In addition to being listed by milepost in the railroad's timetable, block limits are delineated by conspicuous signs along the tracks. Every portion of controlled track belongs to a block, as blocks are laid out back-to-back along the entire length of the rail line.
For example, a 30-mile (48 km) length of main line track may be divided into three blocks, Anna, Bess, and Cloy, each 10 miles (16 km) long. At milepost 10, there will be a sign displaying the end of Anna block and the beginning of Bess block, and a similar sign between Bess and Cloy. A train authorized in Anna through Bess blocks must stop before Bess block ends and Cloy block begins.
For example, a length of track may consist of three blocks. Anna, Bess, and Cloy. There is a siding at Bess, and this siding is specified as Bess Siding. Both Bess and Bess Siding blocks extend to the switches at either end of the siding. Anna and Cloy blocks begin on the other side of their respective switches. Neither Bess nor Bess Siding blocks may be occupied without authority from the train dispatcher.
In this example, a train may have authority straight through from Anna through Cloy. Or, it could have authority in Anna through Bess Siding, in which case it must throw the switch between Anna and Bess and enter the siding--this will usually be done to clear up the main track in preparation for an opposing train to pass. After clearing its authority in Anna siding and the opposing train passes, it might then get authority from Bess Siding through Cloy, in which case it will throw the switch between Bess and Cloy and reenter the main track.
Lengths of DTC blocks vary but usually take about 10 minutes to traverse. The siding itself is a considered is not considered a DTC block per se, but in some situations sidings must not be occupied without authority from the train dispatcher. However in DTC territory all sidings operate as non-controlled, restricted speed track and there is no safety consideration in a train occupying it without central authority to do so, however for traffic flow purposes the dispatcher need to track which sidings are occupied. It would also be possible for a train to have authority from Anna through Bess Siding through Cloy. In practice, though, this is never done, as there is a speed disadvantage to having the train traverse the slower siding track (including having to stop to throw the switches to enter the siding).
While most DTC blocks are laid out back-to-back, the blocks actually do not touch at sidings. Instead, there is a "gray area" over the siding's switches, and the main track's and siding's DTC blocks begins at approximately the clearance point of the switch. The area over the switch must not be occupied unless authority is granted in blocks on both sides of the switch. For example, a train authorized only in Anna block must stop at the End Anna Block sign, which is a short distance from the switch points. A train authorized in Anna through Bess or in Anna through Bess Siding may proceed past the End Anna Block sign, over the switch, and then past the Begin Bess (or Bess Siding, as applicable) Block sign.
Blocks are typically named after a distinguishing feature within the block, such as a station, town, or river.
Read more about this topic: Direct Traffic Control