09/01/13

Fixed Block Systems

Looking back over the past few decades, railway signaling technology has been based mainly on the so called “Conventional Fixed Block System” (ref1, ref2) principle. Traditional signalling systems are based on fixed blocks: the railway is divided into sections of track, which are separated by signals. A train is not allowed to enter a given track section (=block) before the preceding train has cleared it. This system has a number of disadvantages, one being its lack of flexibility: the block size is the same for all trains regardless of their speed and braking performance. Thus the big safety distances required by fast trains are imposed on slower trains as well. Obviously this reduces track capacity.

The fixed block technology inherently imposed a service limitation because of the need to reserve buffer block(s) for train separation. With increasing patronage, demand grew to achieve higher line capacities on existing rail infrastructures. In order to realize this requirement without major upgrades to the rolling stock and rail infrastructure, intelligent signaling and train control systems have become a crucial technology for the new age of rail systems and services. The distance-to-go principle has therefore been developed on the “Fixed Block System,” which provides flexible control of the buffer block(s) for train separation.

Further to that, the “Moving Block System”, which also operates on the distance-to-go principle, has evolved. Moving block systems require less wayside equipment than fixed block systems. They provide considerable cost reductions for personnel and maintenance due to a strong reduction in way-side equipment.

Moving Block Systems (CBTC = Communications Based Train Control)

A moving block system (often called CBTC = Communications Based Train Control) does not require traditional fixed-block track circuits for determining train position. Instead, it relies on continuous two-way digital communication between each controlled train and a wayside control centre. On a moving block equipped railway, the line is usually divided into areas or regions, each area under the control of a computer and each with its own radio transmission system. Each train transmits its identity, location, direction and speed to the area computer which makes the necessary calculations for safe train separation and transmits this to the following train. The radio link between each train and the area computer is continuous so the computer knows the location of all the trains in its area all the time. It transmits to each train the location of the train in front and gives it a braking curve to enable it to stop before it reaches that train. In effect, it is a dynamic distance-to-go system. As long as each train is travelling at the same speed as the one in front and they all have the same braking capabilities, they can, in theory, run as close together as a few metres (e.g. about 50 metres at 50 km/h). This, of course, would contradict the railways safety policies. Instead, one safety feature of fixed block signalling is usually retained - the requirement for a full speed braking distance between trains. This ensures that, if the radio link is lost, the latest data retained on board the following train will cause it to stop before it reaches the preceding train. What distinguishes moving block from fixed block is that it makes the block locations and lengths consistent with train location and speed, i.e. making them movable rather than fixed.

Future Systems

The future signaling and train control system will very likely be a radio-based moving block system using a secure wireless data communication network to continuously track train location, speed and running direction. The subsystem would be required to utilize proven communication technologies to ensure data security and allow for interoperability with other systems. The system must also deliver robust performance under an adverse environment of high radio traffic and electromagnetic noises. It is also essential that the future system offers a low lifecycle cost and is able to overlay on any existing systems to facilitate system replacement.

Current systems: A complex railway signalling, train control and communication environment and interlocking based on standard data network

Far Vision 1: Convergence of rail traffic and road traffic managements: Intelligent Transportation Systems

Far Vision 2: Driverless ATC - make rail network safer, more reliable, and more productive than ever before without a driver

Central Office - ATS (Central ATC functions)

Wayside Subsystem - ATC  (safety-critical wayside functions)

On-Board Subsystem - ATO/ATP (Driverless ATC’s automatic train operation)

Train control systems ensure safe & economic operations respecting the characteristics of the mode:

The subsystem’s vital Vehicle Fault Detection and Response component detects on-board failures—in braking, propulsion, communication, doors, and more--that could occur and instantly responds with an appropriate countermeasure.

And, the entire subsystem features a fully redundant hardware and software architecture, so that if one unit fails, a backup unit can take over that particular control of the train. The result is increased reliability and availability every hour of every day.

More recently, the words electronic, communication-based and software-intensive safety critical systems were joined in the rail industry’s vocabulary by open-standards, platform independent, interoperability, interchangeability, integrated diagnostics and logistical support, and many others previously reserved for high technology fields like the telecommunication, military and aerospace industries.

Automatic Train Control (ATC): The system for automatically controlling train movement, enforcing train safety, and directing train operations. ATC must include ATP, and may include ATO and/or ATS.

Automatic Train Operation (ATO): The subsystem within the automatic train control system that performs any or all of the functions of speed regulation, programmed stopping, door control, performance level regulation, or other functions otherwise assigned to the train operator.

Automatic Train Protection (ATP): The subsystem within the automatic train control system that maintains fail-safe protection against collisions, excessive speed, and other hazardous conditions through a combination of train detection, train separation, and interlocking.

Automatic Train Supervision (ATS): The subsystem within the automatic train control system that monitors trains, adjusts the performance of individual trains to maintain schedules, and provides data to adjust service to minimize inconveniences otherwise caused by irregularities. The ATS subsystem also typically includes manual and automatic routing functions.

Communications-Based Train Control (CBTC): A continuous automatic train control system utilizing high-resolution train location determination, independent of track circuits; continuous, high capacity, bidirectional train-to-wayside data communications; and trainborne and wayside processors capable of implementing vital functions.