Necessity Mothers RETB Reinvention.

Radio Electronic Token Block was born out of necessity in the early Eighties. In 1978, a huge storm destroyed the overhead pole route on the Far North Line from Inverness to Wick/Thurso. Restoration was out of the question economically so how to solve the problem and keep the lines open? The then BR S&T Department in conjunction with BR Research instigated the design of a radio bearer system to transmit the bell and control signals generated by the single line token instruments over the air. In effect, the radio had to mirror the pole route. It sounds simple but was actually quite a challenge.

The kit was duly manufactured, installed and commissioned using the then NRN frequencies of 104-138MHz. It worked well after a few teething problems but the high operating costs of signalling the line remained. Could something more effective be devised? So came about the concept of an electronic token.

If complete radio coverage of the line could be provided - with a guarantee that radio signals at passing loops would be robust - together with an in-cab control unit, then it should be possible to transmit 'electronic tokens' to and from trains under the control of an SSI interlocking and associated control console. Designs and trial equipment were assembled which quickly proved that it was feasible.

Key to the project's success was the design of the radio system, with train radios needing to be identical to the Band III sets (196-206MHz) then being introduced under the NRN programme. The radio infrastructure had also to be compatible and a chain of radio stations was established that required no point-to-point radio or landline links. This took the form of base station>repeater station>base station>repeater station>base station and so on until the end of the line is reached.

Each base station is allocated its own Band III channel. Once the token signal is sent from the control point to the first base station, it is transmitted on-air and received by any trains in that section as well as the first repeater station which changes the signal to the channel of the second base station and repeats it on. Received by the second base station, the signal is again broadcast to any trains in that section as well as to the second repeater. So the process continues to the end of the chain. To guard against equipment failure that would break the chain, the end station is provided with a landline link back to the control point, thus allowing the chain sequence to be fed from the far end. So emerged Specification BR1654 and later the associated Group Standard GK/RT0054.

The way forward

As well as the Far North lines to Wick/Thurso and Kyle of Lochalsh, conversion to RETB operation also took place on the West Highland Line, the Cambrian and Ipswich-Lowestoft (East Suffolk) routes. Storno, a Danish radio company - also supplier of the essentially similar kit for the NRN - provided all equipment for the RETB radio infrastructure. But Storno was acquired by Motorola in the late 1980s and obtaining equipment became more and more difficult. Eventually, a last buy of spares was made but, over time, these have been used up and the systems are becoming impossible to maintain.

The solution for the Cambrian lines has been to replace the RETB with ERTMS Level 2 and use this as the UK test-bed for the ETCS equipment and the GSM-R radio when used as a signalling data bearer. It will also provide experience for the fitting of trains. However, due to the immediate equipment obsolescence challenge faced on Scotland's RETB routes, a medium-term life extension solution was required.

The life extension programme

With performance deteriorating, remedial work started in 2006 on both the West Highland and Far North lines to improve all the obvious deficiencies. Network Rail's Glasgow office engaged Babcock Rail as managing contractor to coordinate and control all the necessary work. Initial activities concentrated on

• renewal of aerials and feeder cables
• testing and upgrading of earth systems
• provision of new or refurbished equipment rooms with improved heating and ventilation
• renewal of some base station battery chargers
• putting in place an improved maintenance regime

In addition, in order to increase operational effectiveness, improvements to radio coverage have been achieved on the Far North line through a number of measures including

• the relocation of a trackside radio site at Achnasheen to a hilltop site at Glen Carron
• re-engineering the coverage from the mast at Tain
• development of a 'cell enhancer' to increase the coverage at Inverness Station, designed to capture and redirect radio signals to small areas.

Operational reliability improved but a full solution was not possible until a supply source of new base station and control equipment could be found.

The replication challenge

The radio equipment parts are nearly 30 years old and the challenge has been to find a specialist firm that could develop a base/repeater station and a RETB control rack that replicated the original Storno kit. Network Rail in Scotland dug out the original technical and functional specifications and updated them to incorporate a number of features that could not have been achieved back in the 1980s.

Rising to the challenge came Comms Design Ltd (CDL) based at Wetherby in West Yorkshire. They undertook to design a new base/repeater station and signaller's console as well as developing a remote condition monitoring system that would enable faults and operational problems to be easily diagnosed.

The products are now real and going through an extensive testing programme centred on Banavie - the West Highland Line control point - plus five radio sites at Banavie signal box (launch signal), Banavie Hill Top (base station), Locheilside (repeater station), Crianlarich (landline link to launch signal on to the Oban section) and Arisaig (base station and far-end landline link). The complete package incorporates a health monitoring system to give an early indication of any performance problems.

As is the case with most product development and reverse engineering projects, a number of unforeseen issues emerged surrounding the ability to integrate elements of the legacy system with its modern-day replacement, impacting on the operational performance of the system. Non-conforming data produced by the existing control equipment was causing the repeaters to remotely switch off whilst timing problems associated with the interface to the SSI and in-cab equipment resulted in some token telegrams being lost.

Resolving these problems demanded innovative thinking from the Scottish engineering team and took a great deal of time as they have had to be worked out from first principles. However, the trial has been ongoing for 18 months, bringing a noticeable improvement in quality without a single equipment failure. The new audio console being trialled at Banavie has received glowing reports from the signallers. The health monitoring system is already showing its worth but will not reach full potential until all the base station equipment is changed over.

Type approval certificates have now been issued for the various types of new radio equipment. The trial is coming to an end and Network Rail in Glasgow has set about producing a roll-out plan for all of Scotland's RETB lines.

RETB base station rollout

A continuation of the development phase includes

• extending the West Highland Line trial to obtain associated product approval for the use of the newly-developed base stations and signaller's console on the Far North Line system
• producing the detailed designs required to implement the remaining replacement RETB base stations on the West Highland and those on the Far North lines. The system serving the West Highland includes the White Corries site which, at 1100 metres, is the highest piece of Network Rail infrastructure
• obtaining final product approval for use of the newly-developed base station cell enhancer to improve the radio coverage at Inverness and thus improve the operational effectiveness of the system at that location.

RETB control rack renewal

As well as the base stations, the life extension programme includes the development and implementation of a new RETB control rack to facilitate and improve the integration of the signaller's voice communication with a variety of radio interfaces. CDL has proceeded with this development in parallel with the base station upgrade work. It will however require a separate product approval and must be independently safety verified in line with the requirements for equipment being introduced into service on the operational railway.

The new control racks will therefore be available before the rollout of the base stations start and the latter can be progressively connected as they become ready for service.

System retrospective

The life extension of this system comes out of necessity, just like the original need back in the 1980s. Network Rail, Babcock and CDL engineers have worked hard to find a solution to the problem of obsolescence and unreliability. As with all modern electronic systems, there is a continual need to ensure that sub-components are available from industry or that modern equivalents are backward compatible to ensure the continued supportability of the system is maintained into the future. The train radios remain unaltered and that includes the ability to fit steam engines with radios when they are used on tourist trains.

The revamped system, with its remote diagnostic facility, should perform well for many years and thus stave off another equipment renewal crisis that could trigger suggestions that the overall economics of the lines might mean closure.

So what of RETB - did it ever reach its full potential? The answer is 'no' - many more lines could have been converted to RETB operation if more money had been put into ongoing development. The concept of operating secondary lines without any lineside cabling and minimal signalling infrastructure has to be appealing. The control and monitoring of level crossings as part of RETB was never properly resolved and this would need pursuing.

Ironically, the financial situation with ERTMS Level 2 is such that equipping secondary lines might never be an economic proposition. Maybe Level 3 will be more attractive but a low-cost version (Level 4?) might provide the answer. Early proposals as produced by the Germans show a remarkable similarity to RETB so perhaps the wheel will come full circle.

The author offered his thanks to Ian Findlay, Ian MacDonald and Alan Simpson of Network Rail, Glasgow for their help in preparing this article.

Our own Iain MacDonald, committee member and former signalman adds the following comments.

It appears like some worthwhile improvements are taking place, but still all transmissions and token exchanges are going through one desk. Banavie has two for less route miles, but it's the length of the sections that rob the line of flexibility more than the signalling system. More loops are definitely needed .e.g. Helmsdale to Forsinard, two trains an hour so if things are a bit out of course and if a train arrives at each end at the same time one of them has to wait half an hour until the other comes through the section. A loop mid way would mean both trains leaving Helmsdale and Forsinard at the same time crossing each other at loop about 13 miles from each point. The same could apply with a loop mid way between Muir of Ord and Inverness.