It is now 40 years since the first Radio Electronic Token Block (RETB) system was introduced. As is so often the case, it had its origins in an urgent problem caused by a major storm in the north of Scotland that destroyed significant elements of the overhead pole route on the Far North line from Inverness to Wick and Thurso. This was in 1979.
This long, lightly-used line was seen as vital by the local communities but the cost of replacing or cabling the pole wires could never be justified. The line was controlled by traditional token instruments located at stations where passing loops existed which needed communications connectivity for them to be operated. The only means of providing a train service in the short term was to use the train staff and ticket system, which was both clumsy and expensive, often involving road vehicles to transport the staff to the adjacent signal box if the sequence of trains changed from the timetabled order.
The line had only just escaped being totally closed under Beeching as it was very expensive to operate with only modest income. Chris Green, then general manager of Scotrail, needed a solution to reduce cost and increase revenue.
Could there be some other way of providing a communications path? The then British Rail (BR) HQ S&T department and the BR Research group at Derby put their heads together and initially designed a system where the bell signals and token instrument controls could be sent over a radio link. The technology was a quasi-sync radio system with block interface control units that converted the DC block signals into audio frequency that were modulated over the air waves. This was successfully deployed and, while it enabled the line to resume normal working, it did nothing to reduce the costs of operation. Could radio technology be used to operate the line from a central location?
RETB principles
RETB works by having a chain of radio transmitting (base) stations on hilltop or high-ground sites along the route concerned interspersed with radio repeater locations, normally sited at one of the passing loops. A radio signal sent from the first base station gives radio coverage for between 10-20 miles of line and which is picked up by the first repeater station. This repeater transposes the signal into a different frequency and sends that signal out which is picked up by the second base station, which then broadcasts that signal to the next section of line. This second broadcast is picked up by the second repeater which again transposes the signal to a third frequency and transmits it on to the third base station.
This chain of events continues until all the line is covered. The repeater system means that no cable connection is needed to feed into the various base stations and thus no lineside cabling is required on the route. To guard against a break in the chain, a rented landline connects the far end site back to the control point so that token control data can be sent in the reverse direction.
The token control coincided with the introduction of Solid State Interlocking (SSI) and resulted in an SSI interlocking being installed at the central control point. This was the first application of SSI and preceded the first main line application at Leamington Spa. The SSI is programmed for the route from which a signaller’s console enables electronic tokens to be issued and transmitted into the radio chain. Every piece of rolling stock that uses the line, including yellow plant, must be equipped with a mobile radio and a cab display unit on which the tokens are displayed. The signaller knows the rough position of every train by receipt of verbal messages received from the driver normally given at the passing loop locations and, under the control of the SSI, can issue a token for a train to go from one passing loop to the next. The SSI prevents the issue of any conflicting token. Once the train arrives at the distant passing loop, the driver contacts the signaller and the token is retrieved. The system relies on verbal messages between signaller and the train drivers but normally a signaller can control up to 20 train movements dependant on traffic levels.
The passing loop points are normally set for left hand running into the loop. There is no facing point lock, but they are controlled by train operated movements. When a train leaves a loop, it runs through the points the wrong way and pushes them over to the reverse direction. Once all wheels have passed, a stored energy device returns the points to the normal facing direction. A speed limit of 15mph over the points ensures safe operation but increases journey times. This speed limit is an impairment for reducing journey times and Network Rail is investigating whether the points can be changed to powered operation under the control of the token that has been issued
The radio equipment was originally supplied by Storno, a Danish manufacturer but subsequently acquired by Motorola, using the frequency bands associated with the BR National Radio Plan (196-206MHz). Eventually, Motorola ceased the supply of radio equipment and thus the source of spares dried up and a new supplier had to be found.
The system was first introduced on the Kyle of Lochalsh line in late 1984 and on the Far North lines to Thurso and Wick in 1985. It was deemed a success. Both routes were initially controlled from a centre at Dingwall which was subsequently moved to the Inverness signalling centre. Later it was deemed suitable for the West Highland Line from Helensburgh to Oban, Fort William, and Mallaig with a control centre at Banavie just outside Fort William. This involved BR buying a hill top for the base station at White Corries using thermocouple gas generators for power, and later converting to solar panels and wind generators.
The system was also deployed on the East Suffolk Line from Ipswich to Lowestoft and on the Cambrian route from Shrewsbury to Aberystwyth and Pwllheli in Wales. Both these have been replaced. The East Suffolk line has many level crossings of varying automatic types and linking RETB to level crossing operation proved difficult. The reliance on radio signals for strike in commands to bring barriers down and for public telephone usage was unreliable so local cabling was found to be necessary and replacement with conventional modular signalling was eventually decided. As is well known, the Cambrian line was selected to be a test bed for ETCS technology.
In the early days, RETB had its reliability problems often necessitating a resumption of train staff and ticket working. Some of this was due to inadequate radio coverage. Later on, a change of frequency band became necessary because of European bandwidth regulation. Both aspects have caused a total rebuild of the systems in Scotland which are described below. Two later developments have been the addition of the Train Protection and Warning System (TPWS) to prevent trains entering a single line section unless they are in possession of a token, and the introduction of a ‘Request to Stop’ system used by passengers at rural stations.
Upgrading the system
By the early 2000s the radio equipment was ageing. The radio coverage needed enhancing and general life extension work to replace power feeds, aerials, and control racks was becoming necessary. Network Rail engineers in Glasgow produced a project plan to bring the systems back up to spec. The design and manufacture of new base station and repeater equipment was given to Comms Design Ltd (CDL) from Harrogate in Yorkshire. Additional base station sites included a ‘cell enhancer’ to better cover the immediate Inverness station area. Also included within the contract was the replacement of much of the radio element of control equipment at Banavie and Inverness, including the audio console. The train borne equipment had also to be changed to accommodate the frequency change and the opportunity was taken to design a combined radio transceiver and cab display unit showing the token issue and exchanges. The SSI interlockings were also renewed. All this was completed by 2010, however it was recognised that further changes would be needed to accommodate European standardisation of European digital TV broadcast bands
Radio frequency change
RETB used the same frequencies as the National Radio Network (NRN) in the 196-206MHz band. This part of the radio spectrum was being re-allocated for digital TV services meaning that RETB would require a new set of radio channels. While the NRN has been superseded by GSM-R, this was deemed unsuitable for cost effective RETB operation. Following negotiations with OFCOM, Network Rail was allocated a total of 20 radio channels within a 2.4MHz grouping within the 180.4-190.8 MHz band to support RETB operation where propagation characteristics would be broadly similar to the earlier frequencies. These frequencies remain to this day and the RETB routes are the only ones in the UK not equipped with GSM-R
The change meant the re-engineering of all the radio elements within the system and a contract to engineer and project manage the radio system upgrade was awarded to Telent, which had the necessary experience in radio network design.
The redesigned system
A starting point was to undertake accurate coverage measurements before doing the detailed planning of new or changed base station sites. This work involved the use by engineers of portable backpack equipment. Riding on scheduled passenger trains, they were able to measure radio signals from selected base station sites along the route. Decisions were then made as to how many existing aerial towers could be used and how many new sites would be needed.
The survey work confirmed that most of the original RETB sites and those additional ones provided during the 2010 upgrade could be re-used. Four additional sites were required at Connel near Oban and Mallaig on the West Highland line and at Kyle of Lochalsh and Balnacra on the Kyle line. Ruggedised antennae were installed at exposed sites to ensure the wild weather conditions did not impact on RETB performance.
Telent used the nominated sub-contractor CDL to supply new equipment for the 48 base station and radio repeater sites. It was also necessary to replace the train borne radios, so along with on-track machines and plant, CDL also supplied 220 new train radios for the Class 156 and 158 DMUs based in Glasgow and Inverness and to various locomotives needed for freight and engineering, including the sleeper service to Fort William. Provision was also made for fitting steam locomotives that powered seasonal tourist trains. Since the trains operate on other than RETB lines, GSM-R equipment was also needed, thus necessitating two radios in the cab. All work was completed by 2015.
Automatic channel selection
Proving the re-engineered system required extensive testing throughout the RETB routes. Another new feature was the auto tuning of the train borne radio to eliminate the need for the driver to manually change channels. At journey commencement, the driver registers the radio with the appropriate signaller to ensure the radio does not auto tune to the wrong signaller’s position. Along the line, the radio will automatically search for a new radio channel when the signal strength of the original channel reduces. Grandfather rights allowed this change in functionality of the system and thus did not need change to the operating rules.
TPWS and other enhancements
An enhancement to the original RETB design has been the introduction of TPWS to provide emergency braking should a train fail to stop at the end of a token section. The TPWS locations are fitted with radio receivers that ‘read’ the token exchanges on the system and transmit ‘stop’ commands by radio in the event of a train being detected without an appropriate token.
Also replaced were the audio consoles for the signallers at Banavie and Inverness. Uninterrupted Power Supplies (UPS) to give 72-hour continuity of operation are installed at all control and radio sites.
Linking to passengers
The RETB routes are in remote areas and very often there are no passengers to join a train at the stations. To avoid the driver having to slow down just in case a passenger is waiting, Request Stop units have been developed by Park Signalling linked into the RETB network. On the platform, a unit is provided that gives the time of the next train services, usually in both directions. A button is then pushed to request the train to stop with this then being posted on to the driver’s RETB token screen. Still under trial, it is expected that these units will be rolled out to all stations on the lines.
What of the future?
RETB has stood the test of time and later investments have made the system more reliable. The system has proven to be a cost-effective way of keeping these lines open. The restriction of needing a captive train fleet is one downside but the operating authorities in Scotland have faith in the system. Whether RETB ever expands on to other routes is questionable. Network Rail is looking at the use of satellite-based technology with train derived positioning which would remove the land-based radio network. Trials are already underway using the ‘Starlink’ network to provide at-seat internet connectivity to passengers using these lines.
Thanks are extended to Alan Ross from NR in Scotland for providing additional information on the RETB technology and operation.
Lead image: White Corries Radio Site.