The re-signalling of London Underground’s (LU) sub surface lines under the management of Transport for London (TfL), collectively known as the 4LM (Four Lines Modernisation), has been well documented in Rail Engineer with updates regularly given as the project has progressed.
Now reaching the final stages, it is appropriate to record and remember the signalling that has or will shortly be replaced. A visit to the signal boxes at Harrow on the Hill and Rayners Lane at the northern end of the Metropolitan Line was arranged by the IRSE London & SE section and a fascinating visit this turned out to be.
The geography
Harrow on the Hill box manages the controlled signals, designated ‘JB’ in the Harrow station area by means of the main box frame and two ‘slave’ boxes which are remotely controlled, at Harrow South and Harrow North plus the area at Northwood using a push button panel.
Outside of the controlled area signals, which are identified by the signal box code, there are automatic sections designated by an A prefix to the signal number, between Preston Road (the boundary to the current extent of the 4LM system) and towards Watford South Junction where the lines towards Amersham and Watford diverge, which is under the control of Rickmansworth box.
Harrow is also the junction for the Uxbridge branch with its grade-separated dive-under bridge. There is a short automatic section to Rayners Lane where the signal box there controls the junction with the incoming Piccadilly Line and onwards to Uxbridge. These two boxes have been adapted to cope with a number of unique operational situations which the 4LM system will have to replicate.
Of note is the boundary between the traditional signalling and the 4LM Communication Based Train Control (CBTC) system. Up until the stage when the 4LM system reached Preston Road, the changeover always took place at a platform with passengers unaware of the change process, but this boundary was different. There are four tracks, these being the northbound and southbound fast and slow lines. On the slow lines, the boundary changeover takes place in Preston Road station, so again passengers are unaware. The fast lines however have no platforms and the changeover must take place (albeit temporarily until the stage at Harrow is commissioned) by bringing the train to a stop at a boundary signal and carrying out the changeover either by engaging the southbound train into the CBTC system or by reverting to ‘legacy’ signalling under driver control for a northbound train, whereby the train can re-start.
Harrow on the Hill
The signal floor is perched three floors above the station and dates from 1948. The heart of the signalling is a Westinghouse N frame (readers may be more familiar with the L type frame that existed on British Rail for many years with only one now left – Maidstone East – plus a few more on heritage lines). The N frame was developed with London Underground in mind and has an unusual adaption with three-position miniature levers for signals where two routes are available. The centre position is the normal with push forward for one signalling function with a unique signal number and pull backwards for another function with a separate number. Not all levers operate in both pull and push functions.
The frame was commissioned with the four-tracking to Moor Park in mind but did not come into service until 1962. Subsequent changes have seen the disappearance of the extensive goods yard and a loop line down to the Uxbridge branch and hence there are now many unused levers coloured white.
The Harrow frame is in two sections, the main one being on the left, manually operated with catch handle lever control, with that on the right being a ‘slave frame’ representing routes controlled by the unmanned interlockings at Harrow South and Harrow North which are linked to the main box by multi core control cables. The slave frame levers are operated by compressed air motors, electrically operated from the main frame, and replicate some of its functions. This section does not have catch handles. Mechanical locking exists right through the frame, and the two slave boxes also have mechanical locking for circumstances when local control is deemed necessary. It is somewhat strange to see these levers moving on their own when the signaller is setting routes from the left section.
North of Harrow is Northwood where a crossover and a siding exist. These are controlled from a separate push-button route setting panel. The site is normally in automatic working requiring no action from the signaller. A remote whistle blower is used which sounds in the siding to warn staff when any train movements are to take place.
Harrow box’s illuminated diagram is unusual in that track indications in the diagram are normally lit and go dark when occupied, a sort of fail-safe arrangement. There is no automatic route setting, and all routes and train movements have to be signalled on the levers.
The Underground does not use ‘main line’ train descriptions in conjunction with the signalling but just shows the train routing type, e.g. AO = Aldgate all stations. These descriptions are set up on a separate screen and are carried forward to adjacent boxes by track circuit actuation as the train progresses. The trains have train numbers, as assigned in the working timetable, and these numbers can be changed if required by operating circumstances.
The relay room below the cabin was immaculate with the wiring looms tied neatly in place – a credit to the technicians who maintain it. The signalling supply is generated at the sub stations at 600 volts, 331/3 Hz. A rarity these days is the use of AC vane relays for the track circuits and point detection to ensure immunity from the DC traction power.
Track layouts
South of Harrow on the Hill, the Chiltern Line trains have a separate pair of tracks down into Marylebone station which are not electrified. From just south of Harrow station, the northbound Metropolitan line joins the Chiltern route and into Platform 6 while Metropolitan trains can reverse south to north on Platform 5. Woe betides any electric train that ventures south of Platform 5 as there are no conductor rails.
Northwards, and after the Uxbridge lines have diverged, the two adjacent fast lines up to Moor Park continue to Rickmansworth and Amersham. They are shared by both Metropolitan and Chiltern line trains. The tracks are TfL operated so the Chiltern trains have to be adapted to meet the operating conditions. There is no AWS or TPWS, but train stops are installed in line with TfL practice. Hence the Chiltern fleet that operates over this section has to be fitted with trip cock equipment.
Train descriptions are a further complication as Chiltern Line trains use the standard Network Rail alpha numeric code for their individual journeys when starting from Marylebone. These are not recognised by the TfL system. A special arrangement is in place to step the descriptions through the joint running section and re-insert these into the Network Rail signalling north of Amersham. The descriptions are shown on a separate screen in Harrow box.
Knowing the whereabouts of every train is important, and the London Underground Trackernet system is being rolled out across all lines as centralised control in introduced. This gives staff in signalling centres and control rooms, and staff with the appropriate mobile phone, a picture of requested track layouts showing every train in that area. The system is also used to feed into passenger information systems and the displays on the Harrow platforms are driven from this source.
The interworking of Metropolitan and Chiltern Line trains can lead to interesting situations but, in an emergency, an alarm can be sent to Marylebone signalling centre with the instruction to stop all trains.
Train communication
The track-to-train radio system on the London Underground has been provided under the Connect contract and uses Tetra technology. Now some +20 years old, it is being updated and modernised. The radio channels are zone related and drivers are called within that zone. A group call facility is available and can be used to stop trains in an emergency. Chiltern trains are equipped with GSM-R, so coverage with that system is also provided.
London Underground has ceased using telephones at signals other than at any location where local circumstances might require one and total reliance on the radio is assured. It was only in April that RSSB’s Industry Leadership Group eventually decided that Signal Post Telephones (SPT) could be phased out on Network Rail’s infrastructure.
Rayners Lane
Two stations along the Uxbridge branch is Rayners Lane, where the signal box is sited just to the west of the southbound platform. The building dates from 1935 and was originally equipped with a Westinghouse push-pull N frame. This frame reached the end of its life in the late 1980s, whence it was removed and replaced by push buttons in the space previously occupied by the levers. These now control a ‘V’ – vertical frame located remotely from the box at the opposite end of the station.
The push button desk controls the line between Rayners Lane station and the junction between the Metropolitan and Piccadilly lines, the turn back siding, the link to the Central Line West Ruislip depot between Ruislip and Ickenham, and the three platforms and sidings at the Uxbridge terminus. Originally the box controlled the stabling sidings at South Harrow on the Piccadilly line route, but these are under the control of the Piccadilly line signalling centre at South Kensington.
Between these controlled areas, a number of stations exist along the route with automatic signalling controlling the Metropolitan and Piccadilly line trains. That said, there is a need for speed control approaching Uxbridge platforms as they are ‘dead end’ and the signalling automatically holds the approach signal at red necessitating a train to slow down before the signal is cleared. This is a legacy from the unfortunate Moorgate incident in 1975 when a train running under clear signals ran through a terminal platform and collided with the end tunnel wall resulting in many deaths. The stabling sidings at Uxbridge are equipped with dot matrix shunt signals. The box-illuminated diagram is conventional with track occupancy shown with red lights.
Going forward
In the near future, these two boxes will be replaced by the 4LM CBTC system as that system gradually takes over. Catering for both Chiltern Line and Metropolitan trains on the line from Harrow to Amersham will involve the introduction of blue signals on the conventional three-aspect signals for Chiltern trains. This novel idea is to give Metropolitan drivers the assurance that signals can be passed at red if the train is under automatic control.
Other new features will be needed on the Uxbridge branch to cater for both Metropolitan trains under CBTC and Piccadilly line trains on the existing legacy system until such time as new trains are introduced with a compatible signalling system. The old boxes have served the lines well and it is only right that some record of their operation is made.
Thanks are extended to the local TfL staff and to Paul Baker from the IRSE London & SE section for organising this fascinating visit and his ‘sanitising’ work to ensure the accuracy of the article.
