DLR signalling  
The fixed-block system
In an automated train system it is important to know exactly where all trains are and to keep them a safe distace apart from each other. Most train systems use a so called 'fixed block' system to do this. This system devides the track into a number of blocks and will only allow a train into a block if the next block is cleared.
fixed block system example
In this example train A can only enter block B (A123) when train B has cleared block C (A125).
This means that there's always a distance of more than one block between two trains. To run a frequent train service like DLR does, the track has to be devided into many short blocks, requiring installation and maintenance of lots of signalling equipment. The number of blocks is limited by the minimum length of a block, which is the distance it takes for a train at full speed to come to a complete stop in the worst possible conditions. The biggest disadvantage of this system is the long distance needed between two trains which limits the capacity of the railway. The original DLR control system was a fixed-block system which could run a 2 minute service frequency.
This system was later replaced by the latest technology: a transmission-based or moving block system.
The moving-block system
On railways using a fixed-block system trains are always at least the lenght of one block apart from eachother. So even at low speed it's impossible to decrease the distance between two trains. The real safe distance between two trains is the distance needed for a train to come to a stop before hitting the train in front of it, which is much shorter than the fixed-block lenth and even shorter at low speed. The moving block system allows trains to travel much closer to eachother. Instead of cutting a piece of line into fixed blocks, the train itself and a part of the line in front of it and in its back becomes a moving block which no other train can enter. The length of this block (also called the safety zone) depends on the braking distance of the train which in its turn depends on the speed of the train and the track conditions locally.
For a moving block system to work, the system needs a reliable train position and speed to calculate the safety zone surrounding the train. The system onboard the train itself continually calculates its own position and transmits it, along with other data like speed, direction and other onboard status data, to the wayside systems. In return the wayside systems transmit data like maximum permitted speed and the current target point of the train, which is a point along the line that can be reached safely without any obstructions in the way. By advancing the target point of a train along the way, the train is safely guided to it's next stop at a safe speed and a safe distance to the train in front of it. This way trains can run much closer together making it possible to run trains at a much higher frequency, even as short as one minute apart.
When replacing the old system, DLR had to look out for a new but yet operational system. The chosen system was Seltrac, supplied and installed by Alcatel of Canada. It was already in use on railwaysystems in Vancouver, Detroit and Toronto where it had proven to be a reliable system. Seltrac is a transmission-based moving-block automatic train control (ATC) system, combining both automatic train protection (ATP) and automatic train operation (ATO). The seltrac system consists of 3 main parts ('primary control levels'). Two of those are vital for running the trains, the third system provides the human interface to the system so operators can regulate the service and change scheduling. As Seltrac is an automatic train operation system, the system can run without human help, some people even say the system runs better if no one touches it...
The two vital systems are the ones on the train itself and the fixed wayside system. The onboard system (the 'telemetry and vehicle onboard controllers') is the least intelligent part of the whole Seltrac system (unless my neighbour sits down at the controls in the control-centre...) It consists of two main systems; one for reliably calculating and transmitting the current position and speed of the train and the other for controlling motoring power and brakes so the train moves towards the received target point at the received maximum speed.
The most important part of the system is the 'Vital vehicle and track equipment level' ,which controls all movement of the trains by calculating a safety zone for every train from the data received from it, calculating a safe targetpoint and speed and transmitting it back to the train. It also controls the track equipment (like points) and monitors transmissions from trains for problems reported by the onboard system or even complete loss of communication.

Below is a blockdiagram of the complete Seltrac system used by DLR, including a fixed-block backup system for tracking non-communicating trains.
Seltrac overview
Vehicle and track equipment control
As you can see in the block diagram, the heart of this part of the system is the VCC (Vehicle Control Centre). The DLR system is controlled by three VCC's, one for the western part from West India Quay to Bank and Tower Gateway, VCC2 for the middle part from Island Gardens to Stratford and VCC3 for the route from Poplar to Beckton. A VCC consists of three computers which continually monitor eachother for correspondence. The VCC communicates with the trains via track loops, working as an antenna(using 36 and 56 kHz frequencies), which are up to 3.2 km in length and transposed every 25 metres. These transpositions do not interfere with the transmissions but can be detected by the onboard systems of the train and are used to determine the position of the train. Each VCC can control up to 15 loops. Once the VCC has received the exact position and speed of the train, it calculates a safe targetpoint and speed for the train and sends this information back to the train via the same trackloop. The VCC holds a geographical map of the complete railway including the position of points, stations and trackloops, track-gradient, maximum line speed and zones with increased braking distance as a reference for controlling train movement. The trains are controlled by advancing target points along the route. A train has a target point that looks for the next station or set of points or train but always maintains a safety distance of 50 metres. The target point is specified as a loop number and will be approached by the train at maximum line speed. If the target point is not advanced in time, the train will slowly brake and stop just before the target point is reached.
In case the VCC loses contact with a train, it loses the information about its current position and it can no longer control the train, which could be a dangerous situation. The system onboard the train detects this situation and puts the train to an emergency stop. That solves the problem of the train running unsupervised but the VCC still can not track the train when it is manually removed from service (by a driver onboard the train). To solve this problem, DLR have added an extra system to the Seltrac system; a fixed-block axle counter system. When a non-communicating train passes an axle-counter head (above the 14664 sign on the picture below on the right) it's location is known and the VCC is able to advance the target-points of following trains. The VCC keeps tracking the defective train this way until it is cleared from the running rails into a depot for repair.
Telemetry and onboard vehicle controllers
Each DLR vehicle is equipped with a Vehicle On Board Controller (VOBC) which controls all onboard functions like determining the exact position of the train, motoring and braking and communication with the VCC. The VOBC determines the coarse position by detecting the transpositions in the track loops. It also uses the transpositions to calibrate the onboard tachometers continually. The tachometers are attached to non powered wheels to minimise the chance of wheelslip and keeping the measurements accurate. In case wheelslip does occur, an accellerometer acts as a backup so an accurate train position is always available. The VOBC communicates all data to the VCC and gets a new target point and maximum line speed back from the VCC. The VOBC uses this data to control the motoring and braking so the train follows the targetpoints at the maximum permitted speed. If the train approaches the targetpoint it applies the brakes so the train slows down and stops just before the targetpoint is reached. If the train threatens to overshoot the targetpoint the emergency brakes are applied and manual intervention is needed to get the train moving again. Each VOBC is designed to support up to three car-operation and any VOBC onboard a multi-car train can control the train.
This part of the system is not vital for the functioning of the railway. It provides the operator interface to the system and the facilities for automatic scheduling and regulation of service. The SMC (system management centre) passes its data to the VCC using a number of low speed serial links. It allows the operators to regulate the service and sends various equipment status alarms and reports back to the operators. The SMC can take over the functions of controlling track equipment and tracking train position in case of a failure of the vital control level using an emergency gateway to the station controllers. In case of SMC failure, there is a direct emergency link to the vital control level. In this case the trains run automatically without service regulation.

Docklands Light railway page / R.P.F. Consemulder / dlrsite@conz.co.uk / 19 august 1999