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.

Timetable and Train Order

See also: Train order operation

The most popular method of working single lines in the United States up until the 1950s. Trains operate by timetable, but a dispatcher can amend the schedule of trains by train order. The dispatcher can also alter the stations at which trains are to meet, authorize extra trains and annul timetabled trains as required.

Train orders are communicated by the dispatcher to station staff by telegraph, or later telephone. Station staff write them out on a form, read them back to the dispatcher, and pass them to the train crews. Two types of train order exist: orders that can be passed to the train crew without the train having to stop, and orders that are regarded more seriously, for which trains must stop so crew members can read them and sign for them as read. A signal is usually provided at stations to inform approaching trains there is an order for them.

Complexity is often added with the concept of superior and inferior trains. Trains may be regarded as superior or inferior by way of class or direction. Thus a passenger train may be placed in a superior train class to a freight train, while eastbound trains may be regarded as superior to westbound trains of the same class, for instance. Considerable judgment is required of train crews to maintain a timetable, comply with train orders, and at the same time avoid delaying any superior train.

One Train Working

One Train Working (with train staff)

See also: Token (railway signalling)

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.

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. 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

Method of working	For use on	Movement authority by	Line proved clear by
Timetable operation	Single track	Adherence to timetable	Adherence to timetable
Timetable and train order operation	Single track	Adherence to timetable or possession of written train order	Adherence to timetable
Track warrant control	Primarily single track	Possession of track warrant	Dispatcher's receipt of verbal advice via radio that previous train has vacated the section
Direct traffic control	Primarily single track	Verbal authority via radio	Dispatcher's receipt of verbal advice via radio that previous train has vacated the section
Time Interval Working	Single or multiple track	Handsignal or clearance of section signal	Period of time elapsed since passage of previous train
One Train Working (with train staff)	Single track branch lines	Possession of train staff (and clearance of section signal, where provided)	Possession of train staff
One Train Working (without train staff)	Single track branch lines	Clearance of section signal	Sequential operation of train detection at entrance to section
Train Staff & Ticket	Single track	Possession of train staff or ticket (and clearance of section signal, where provided)	Overlaid block system or time interval working
Divisible Train Staff	Single track	Possession of train staff or portion thereof (and clearance of section signal, where provided)	Overlaid block system
Electric Token	Single track	Possession of token (and clearance of section signal, where provided)	Possession of token, cooperation between signalmen and signalman visually observing that previous train has vacated the section
No-signalman Token	Single track branch lines	Possession of token (and clearance of section signal, where provided)	Possession of token
No-signalman Token with Remote Crossing Loops	Single track	Possession of token	Possession of token
Radio Electronic Token Block	Primarily single track	Electronic token and verbal permission via radio	Possession of electronic token and signalman's receipt of verbal advice via radio that previous train has vacated the section
Tokenless Block	Single track	Clearance of section signal	Cooperation between signalmen and signalman visually observing that previous train has vacated the section
Absolute Block / Manual block	Primarily multiple track	Clearance of section signal	Cooperation between signalmen and signalman visually observing that previous train has vacated the section
Track Circuit Block / Centralized traffic control	Single or multiple track	Lineside signals	Continuous train detection
Cab signalling (various systems)	Single or multiple track	Cab signals	Continuous train detection
ERTMS Level 1	Single or multiple track	Lineside signals or cab signals	Continuous train detection
ERTMS Level 2	Single or multiple track	Lineside signals or cab signals	Continuous train detection
ERTMS Level 3	Single or multiple track	Cab signals	Train continuously reports its position via GSM-R radio