Signalling block system
Signalling block systems enable the safe and efficient operation of railways 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 for which a danger exists of both head-on and rear-end collision, as opposed to double track, whose 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 US and safeworking in Australia.
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American train order system was often implemented on top of other block systems when those block systems needed to be superseded.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.
Authority to occupy a block is provided by physical possession of a token or train staff that is obtained in such a way that ensures that only one train is in a section of track at a time.
Ordinary train staff sections
Some low traffic lines dispensed with Tickets and became Ordinary Train Staff sections (OTS).
Electric train staff (ETS)
These came in two sizes, large and miniature.
Manual block system
Authority to occupy a block is conveyed to trains by the use of wayside signals manually controlled by human operators following various procedures to communicate with other block stations to ensure separation of trains.
In this system, the occupation of a given section of track between two stations is agreed between its station masters, via telephone. 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. In the Portuguese system, although the authority of train movement on the main lines is the sole responsibility of the stations along those lines, a regulating post oversees them and, in case of disagreement, instructs stations as to how the traffic should be organised. On the other hand, each train timetable indicates all interactions with other trains (e.g., crossings with other trains; trains that they overtake; trains that overtake them) clearly marked at the stations at which those interactions should occur. Any deviation from that—arising, for example, from delays or extra trains—must be provided to the train crews in writing. Despite the general practice that, when two trains cross, they both stop at the nearest station, this system allows for good average speeds for fast trains similar to those on an automatic-signalling line. 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). Such delays would not happen, at least not for the same reason, on an automatic-signalling line.
In general, the system dictates that a block is assumed to be closed; that is, permission must be obtained before a train is allowed to enter a block at one station en route to the other. However, in France, on multiple tracks, the block is usually open in unidirectional track sections. That is, after a station confirms that a previous train has vacated the block, the next train travelling in the same direction can immediately enter the block, with the station master at the entry station informing the exit station of the time that the train entered the 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.
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|
|Telephonic block (closed block)||Primarily single track||Hand signal presented by the station master||Station master observation that train has passed complete and telephonic dispatch to previous station. The next train can only be dispatched with the next station station master authorization|
|Telephonic block (open block)||Multiple track||Hand signal presented by the station master||Station master observation that train has passed complete and telephonic dispatch to previous station. The next train can then be dispatched, being the next station station master informed that a train has been dispatched in his direction.|
|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|