Signalling block system

Signalling block systems enable the safe and efficient operation of railways by preventing collisions between trains.

[a] As a result, the number and size of blocks are closely related to the overall route capacity, and cannot be changed easily because expensive alterations to the signals along the line would be required.

Any block system is defined by its associated physical equipment and by the application of a relevant set of rules.

This provides the ability to implement a set of blocks using manual signalling based at these locations.

This method is not authorised for use in many high-traffic railway systems because it is potentially unsafe and highly inefficient.

Trains would make use of a predetermined operating plan known as the timetable which made use of fixed passing locations often referred to as stations.

In UK terminology, this method of working was originally referred to as One Engine in Steam (OES).

Safety is ensured by the interlocking circuitry, and if a track-circuit failure occurs then special emergency working by pilotman must be introduced.

He could then proceed, surrendering the ticket at the other end of the section, and a second train could follow in possession of the staff.

Used on multiple track sections whereby the passage of trains from one point to the next is controlled by instruments connected by telegraph wires.

These messages are conveyed by telegraph instruments with a key that is pressed to sound a bell at the remote signal box.

Such systems, such as absolute block signalling, were developed in the nineteenth century and are still used extensively in Britain and Australia.

For greater safety there can be additional layers of protection; for example, a regulating post, with supervisory powers connected to all the stations in a line; timetable (Portugal); and/or computer assistance (France).

Portugal, Spain and France still use this system on at least some main lines, although the total length of track governed by this system is decreasing rapidly due to its labour intensity and its inherent perceived lack of safety, relying as it does primarily on human communication (sometimes involving more than just the two station masters at each end of the block) and simple railway interlockings at the stations.

In Portugal, the telephonic block was the main safety system across the national railway network until the mid-1990s due to lack of resources.

Thus, it evolved to try to provide multiple layers of safety on busy single-track lines with diverse train types, albeit at the cost of high levels of staffing.

However, if minor delays occur and then proliferate, longer delays can arise as the system's additional safety mode is invoked (i.e. the paperwork-intensive process of updating train-movement instructions to reflect the altered crossing patterns).

This is a system for use on single track railways, which requires neither the use of tokens nor provision of continuous train detection through the section.

[4] The first use of block working was probably in 1839 when a Cooke and Wheatstone telegraph was installed in the Clay Cross Tunnel of the North Midland Railway.

In 1842, William Fothergill Cooke, who had built the Clay Cross system, published Telegraphic Railways or the Single Line in which he proposed block working for general use as a safer way of working on single lines.

Previously, separation of trains had relied on strict timetabling only, which was unable to allow for unforeseen events.