HomeSignalling and TelecomsFond farewell to a favourite power box

Fond farewell to a favourite power box

Listen to this article

The huge modernisation of railway signalling in the UK during the 1960/70s and early 1980s came about through the introduction of Power Signal Boxes (PSB), with many covering significant miles of route. All the BR regions had them and, by the time of the last introduction, most of the trunk routes had been re-signalled, offering big increases in capacity by means of continuous track circuit block sections, as well as an element of centralised control.

However, these PSBs are now coming to the end of their economic life, being replaced by the Rail Operating Centres (ROCs).

The Southern Region was no exception and had a plan to cover its entire geographic area with 13 PSBs. The first and second of these were at Dartford and Feltham, but the third, and by far the biggest, was at London Bridge. This area had been signalled in the 1920s with individual signal boxes at all the major locations, including the notorious Borough Market Junction, where the lines to Charing Cross and Cannon Street diverged.

With the heavy and increasing suburban traffic, an urgent need existed to make permanent way alterations and centralise the signalling. So came about the London Bridge Power Box, opened in stages during the period 1975/6 with due ceremony, the Duke of Edinburgh being the guest of honour.

The box has been in service for 45 years but, in recent years, much of its operating area has been transferred to the ROC at Three Bridges. The summer of 2020 saw the very last area of control at Hither Green transferred, and so ended a remarkable period of operation that had transformed the railway service in SE London.

This article traces the technology of the London Bridge box and the innovative additions that were state of the art at that time.

London Bridge area

The Southern had three divisional areas, SE, Central and SW. The latter was pretty much self-contained, but the South East and Central came together at a number of London terminals, London Bridge being one of them. The new PSB had to span these boundaries, so integrating the management aspects was part of the project.

The entire scope of the re-signalled area is shown on the attached map. The Central division section is on the left, covering the lines through New Cross Gate to Norwood Junction and Clapham, of which most trains ran into the terminus side of the station. The SE division covered the lines to Charing Cross and Cannon Street and out into the suburban area of Kent. To facilitate this split, the box had two control desks, a small one for the Central and a much larger one for the SE. Some train services from the Central division ran through to Charing Cross in the off-peak hours and thus an arrangement was made to transfer these from one panel to the other.

Statistics for the PSB area are as follows:

Route miles – 47.3, track miles – 147.8

Total number of route settings – 926

Point machines – 456, Colour light signals – 547, Junction indicators – 102

Train describer berths – 456

Building the Power Box

It was seen as important that the new box should be located at London Bridge station where finding the right plot of land was something of a challenge. Eventually, space was found at the country end of the station on the west side where the nine terminal platforms were situated. The site was somewhat constricted, which resulted in a long thin design, the ground floor being used for the relay rooms and other equipment areas, including the all-important power supplies, with the operating floor built above on the first floor.

The building would have been designed by the Southern Region architects with construction undertaken by a major building contractor. Security was not taken as seriously then as it is now, so there were no formalised access arrangements. This was made much more rigorous during the building’s life time.

The considerable ground area necessitated a local UHF radio system for maintenance staff to use when communicating with each other inside the box.

Signalling technology

The main signalling contract had been let to Westinghouse Brake and Signal Company, based in Chippenham, which first became Invensys and is now Siemens Mobility.

Computer technology was in its infancy at the time LB box was designed and certainly did not embrace any safety critical circuitry. As such, the main interlockings at London Bridge and the outlying relay rooms were relay-based using ‘Westpac IV’ geographical units. This made the design simpler, although it used more relays and took up more space than a free wired alternative.

Developed in the 1960s, geographical was really a misnomer as it consisted of groups of relays assembled into a number of self-contained modules with standardised wiring, each with a specific function, and then bolted on to traditional racks. Typical modules were a main signal, a shunt signal, a set of points and such like. Around 20 modules were developed, which included some non-vital functions.

The inter-wiring between the modules used multicore cables, with some use of plug couplers, and had to be designed for the specific application. This included the use of some free-wired relays as an interface to external equipment such as track circuits and point machines.

At some of the remote relay rooms, the space required for the geographical racks was somewhat limited due to the size constraints of the buildings, as can be seen from the photograph of Hither Green.

There was much discussion at the time that the high-density routes, with trains likely to be close together moving at relatively low speed, did not justify the provision of 4-aspect signals rather than 3-aspect. In the end, 4-aspect was chosen, which proved a wise decision as the drivers used the double yellow as an indication of the position of the preceding train, thus regulating their speed accordingly and avoiding sudden stops.

The operating floor arrangement was a separate control desk and a mimic panel behind it for both the Central and SE lines. Thus, the signaller could set routes with entrance/exit buttons from a sitting position for most of the time and watch train movements on the panel, where the route setting, track circuit occupancy and train describer berths were displayed.

The panels incorporated mosaic tiles which facilitated an easier change when any future track alterations were made.

Additional signalling features

With the intense traffic in the London Bridge area, the previous signalling had permitted reduced overlap distances to enable signal clearance behind a particular train to take place earlier than would normally be the standard, thus allowing trains to ‘close up’. To have adopted the standard overlap in the new signalling would have reduced the capacity of the line, so a number of facilities were built in to ensure this would not happen.

One was Train Operated Route Release, whereby routes were automatically set, providing approach locking conditions were met. Another was to give the signaller a choice of whether to retain the original overlap distance in heavy traffic conditions.

Additional reversible lines were incorporated into the new signalling, but care was taken not to show clear signals within the driver’s sight line before the actual route into the reversible section was set.

Closing-up signals were another feature, whereby a train, held at a signal because of another one stopped in a platform, was allowed to proceed towards a closing-up signal once the first train began moving. If the first train proceeded as expected, the closing-up signal would clear before the second train got to it, thus saving an average of around 30 seconds.

A novel feature was automatic working at some of the outlying termini and turn back stations. At Hayes, Bromley North, Elmers End and Grove Park, terminating trains would automatically be signalled during off-peak hours into a single platform, whence, once the appropriate track circuits were occupied, the route back would be automatically set and the signal cleared, thus producing some staff savings.

Early use of computers

The train-describer system used cathode ray tubes to show the train reporting number on the main panels, with these being driven by an early design of mini-computer. It was recognised that distributing the TD information to other desk positions on the operating floor, as well as to remote locations away from the box, would be useful, thus visual display units (VDUs) were connected, via a low speed data line, to give a selection of maps showing the various areas of the box and the movement of trains within it.

By keying in either the train number or a signal number, the outlying places could easily keep track of trains. Particularly important were the control rooms at Charing Cross and Cannon Street, where train ready to start (TRTS) buttons and the platform starting signal could be seen.

The Southern Region placed much emphasis on passenger information and this was automated at London Bridge and Waterloo East stations. A computer holding timetable information was set up in the PSB, which linked to the train describer computer to compare scheduled and actual running times of trains. This then linked to an APIS (Automatic Platform Indicator System) computer which drove the platform flap indicator displays at the two stations. Thus, ‘next train’ information could be automatically set according to the timetable with the capability of altering or deleting the displays if trains were running out of course, delayed or cancelled.

This system was refined and much improved for later Southern Region power boxes.

Changing the Track Layout

As with most power box projects, permanent way alterations often accompany a resignalling project. At London Bridge, some 80 stages of layout alterations were made, the major ones being at Cannon Street, Bricklayers Arms and New Cross Gate.

A main objective was to separate out the Charing Cross and Cannon Street trains before arrival at London Bridge, thus eliminating the infamous Borough Market Junction signal box.

The permanent-way stageworks commenced in 1972 and continued through until 1978, meaning that the associated signalling alterations had to be carried out, firstly on the old signal boxes, and later within the new power box. Locking alterations were kept to a minimum, making use of existing cabling as much as possible.

Then and now

So ends an era. London Bridge Power Box enabled a much-improved train service to be achieved. It epitomised the time of route relay interlockings and large display panels, as well as introducing some very welcome new features.

The Southern never achieved its 13 Power Box goal, but other large ones did come along – Victoria (located at Clapham), Three Bridges, an expanded Eastleigh, Ashford – but London Bridge set the trend. It was not perfect, as shortage of finance prevented the ultimate sorting out and expansion of lines in the Borough Market area and the grade separation at Bermondsey. It took the emergence of Thameslink and the Three Bridges ROC to finally get the layout that London Bridge warranted.

Over the years, some equipment within London Bridge would have been upgraded, the computers for the train describer and information systems in particular. Track layouts will have been changed, meaning alterations to the interlockings and control desks and displays, but the basic systems remained in place.

Since 2015, when the new plans for the wider London Bridge area began to take effect, the PSB areas of control have gradually transferred to Three Bridges, with the Central side the first to go completely. Now, in 2020, the last element has moved elsewhere and the box, in terms of operation, has closed.

Technology has moved on over the 45 years. Computer interlockings have replaced geographical relay circuitry, VDU screens with tracker ball control have replaced the big mimic panels and control desks, and computers have totally taken over the intelligence required for train reporting, regulation and information systems.

Nothing lasts for ever, but London Bridge box had a good innings.

Clive Kessell
Clive Kessell
SPECIALIST AREAS Signalling and telecommunications, traffic management, digital railway Clive Kessell joined British Rail as an Engineering Student in 1961 and graduated via a thin sandwich course in Electrical Engineering from City University, London. He has been involved in railway telecommunications and signalling for his whole working life. He made telecommunications his primary expertise and became responsible for the roll out of Cab Secure Radio and the National Radio Network during the 1970s. He became Telecommunications Engineer for the Southern Region in 1979 and for all of BR in 1984. Appointed Director, Engineering of BR Telecommunications in 1990, Clive moved to Racal in 1995 with privatisation and became Director, Engineering Services for Racal Fieldforce in 1999. He left mainstream employment in 2001 but still offers consultancy services to the rail industry through Centuria Comrail Ltd. Clive has also been heavily involved with various railway industry bodies. He was President of the Institution of Railway Signal Engineers (IRSE) in 1999/2000 and Chairman of the Railway Engineers Forum (REF) from 2003 to 2007. He continues as a member of the IRSE International Technical Committee and is also a Liveryman of the Worshipful Company of Information Technologists. A chartered engineer, Clive has presented many technical papers over the past 30 years and his wide experience has allowed him to write on a wide range of topics for Rail Engineer since 2007.
Previous article
Next article


Please enter your comment!
Please enter your name here

This site uses Akismet to reduce spam. Learn how your comment data is processed.