Signalling block system

Signalling block systems enable the safe and efficient operation of railways, so as to avoid collisions between trains. Block systems are used to control trains between stations and yards, and not normally within them. Any block system is defined by its associated physical equipment and by the application of a relevant set of rules. Some systems involve the use of signals, while others do not. Some systems are specifically designed for single track railways, where a danger exists of both head-on and rear-end collision, as opposed to double track, where the main danger is rear-end collision.

A block system is referred to as the method of working in the UK, method of operation in the U.S, while in Australia they come under the heading of safeworking. The objective of this article is to describe the various methods of block signalling used around the world.

Block signalling methods

Strict timetable operation

Trains operate according to a strict timetable, that is cannot leave a station until an appointed time and until any other trains they were to meet at that station have arrived. Rarely used as a block system, as if one train is delayed, all trains it is scheduled to meet are delayed. This can quickly lead to all trains on the railway being affected.

This method is not authorised for use in the UK.

Timetable and Train Order

Popular on single track lines in North America up until the 1980s, Train Order operation was less a block system and more of a system of determining which trains would have the right of way when train movements would come into conflict. 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. Amendments to the operating plan would come from a train dispatcher in the form of train orders, transmitted to the trains via intermediaries known as agents or operators at train order stations.

This method is not currently authorised for use in the UK. A similar system, known as Telegraph and Crossing Order, was used in the 19th century, but after three serious head-on collisions in the 1870s (Menheniot, Cornwall Railway, 1873; Thorpe, Great Eastern Railway, 1874; Radstock, Somerset & Dorset Railway, 1876) its use was condemned.

In North American train order system was often implemented on top of other block systems when those block systems needed to be superseded. For example where manual or automatic block was implemented, train orders would be used to authorize movements into occupied blocks, against the current of traffic or where no current of traffic was established.

One Train Working

One Train Working (with train staff)

If a single track branch line is a dead end with a simple shuttle train service, then a single token is sufficient. The driver of any train entering the branch line (or occupying any part of it) must be in possession of the token, and no collision with another train is possible. For convenience in passing it from hand to hand, the token was often in the form of a staff, typically 800 mm long and 40 mm diameter, and is referred to as a train staff. Such a staff may be a wooden staff with a brass plate stating the section of line on which it is valid, or it may be in the form of a key.

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

One Train Working (without train staff)

A modern variation of the One Train Working system operates without any train staff. On these lines the clearance of the controlled branch entry signal is the driver's sole authority to enter the branch, and once the train has passed that signal, the interlocking will hold it at 'danger' (and the signal cannot be cleared a subsequent time) until the branch service train, on its return journey has sequentially operated two track circuits at the start of the branch. Continuous train detection on the branch is not required. Safety is ensured by the interlocking circuitry, and if a track-circuit failure occurs then special emergency working by pilotman must be introduced.

Electric Train Staff (ETS)

These came in two sizes, large and miniature.

See Token (railway signalling).

Ordinary Train Staff and Ticket (OTST) or (OTS&T)

See Token (railway signalling).

Ordinary Train Staff sections

Some low traffic lines dispensed with Tickets and became Ordinary Train Staff sections (OTS).

See Token (railway signalling).

Telegraph Block

Used on multiple track sections whereby the passage of trains from one point to the next was controlled by instruments connected by telegraph wires. Used extensively in Australia.^[1]

Tokenless Block

This a system for use on single track railways, which requires neither the use of tokens nor provision of continuous train detection through the section. The signalling is designed such that the controlling signals will only allow one train to enter the line. The signalman at the far end of the section must visually check that the whole train has left the section and not become divided.^[2] Examples abound on railway systems throughout the world.^{[citation needed]} The absence of a token does remove the one visible reassurance that the driver has and head on collisions have occurred where the driver of an unauthorised train has proceeded under the mistaken idea that he has authority to occupy the line.^{[citation needed]}

Absolute Block

See British absolute block signalling.

Comparison of block systems

References