S & T Notes - issue 130

SIGNALLING NOTES - Chris. Hall

Defect Rectification

The crossover at the north end of Bridgnorth (no. 29 crossover), referred to in the last article, has now received the concentrated attention of the Department. A life-expired compensator was to be replaced, worn pins replaced and the switch opening reduced as well as straightening up what was referred to as 'Harsley' rodding. The initial expedition to disconnect the worn compensator didn't go all that well - there seemed to be some unexplained discrepancy between the equipment supplied and the dimensions of the existing fitting. After a lot of physical effort to dig out the concrete legs in a snowstorm, this became clearer and representations were made to the surveyor who was, fortunately, part of the working party and, even more fortunately, had been involved in the excavations. There was nothing for it but to put in more effort to restore the equipment to its original, flawed, condition following which some members of the working party booked off and adjourned for refreshment to the Railwayman's Arms. Several rounds later all seemed calm and well again. Following a subsequent visit the next weekend, the crossover now operates smoothly. A compensator is a device placed half way along the point rodding run which transforms a push to a pull so that the expansion of the rodding in the heat of the sun does not cause the switch blades to go out of adjustment (a 100 yard length of rodding might expand by around 1½" in hot weather). The old compensator had slack clearances making the points difficult to adjust.

This left one job still to do at the north end of Bridgnorth: replacement of the location cupboard by the boiler shop. This provides the circuits for the two adjacent track circuits and disc signals and the existing location cupboard was subsiding gently into an adjacent pile of boiler tubes. During routine maintenance in early March (track-lubing or greasing the point slide chairs) however, there seemed to be something missing and, sure enough, the lamp on no. 30 signal (the upper disc of the double disc signal reading out of the Hollybush Road siding) had disappeared. As the damage site was being examined, puzzling how the the lamp had been so neatly disconnected, a furtive whisper gave information as to the fate of the lamp. Collaboration between the P. Way and the Locomotive Department had restored the lamp to a semblance of its original shape. The collaboration was explained by the fact that brand new lamps on this signal had been 'nudged' by the P.Way a while ago, not very long after installation, and a full and frank exchange of views on that occasion had left a lasting impression. It quickly emerged that the P.Way had been doing some shunting and that the lamp had been removed for some swift first aid. It also appeared that these events might be linked! The remains of the lamp were collected and put on the S&T work list and a refurbished lamp has now been installed. That should mean that we'll have no trouble borrowing portable lamps from the P.Way for track circuit testing (and possible re-bonding) in Bewdley tunnel later this month.

Some six man days of the Department's efforts have been directed to broken (and missing) track circuit bonds. We were first alerted to this by a track circuit failure at Bridgnorth on which a temporary repair had been effected by the permanent staff. A tamper had been operating in the Bridgnorth area and had chewed up and spat out two sets of track cables and four bonds near the bypass bridge. The spoor of the tamping machine had also been noticed in the Bewdley area: during routine track circuit testing there in the last two weekends of February more damage was observed. Initially this appeared to be four broken bonds between North box's Up Home and Up Inner Homes but on a subsequent visit to replace these, a further nine broken bonds were found between the banner repeater and the Up Home. A fishplate joint had also mysteriously appeared in the middle of a sixty foot length of rail with no holes drilled for bonds. This was of course a source of some head scratching and made the use of a rail drill unavoidable. The competition to guess the source of the new joint is not eligible to members of the P.Way department and the current rumour is that a broken rail was repaired just before Christmas (confirmed in 'Track Topics' in the last issue). A new sixty foot length of rail (more track circuit bonds to fit) has since been laid.

Track circuit bonds are made by placing the bond wire in a hole in the web of the rail either side of the fishplate and hammering a soft pin, with a groove to fit over the wire, into the hole. This makes a reliable electrical connection between the two rails and bonds are normally fitted in pairs. When replacing track circuit bonds the preferred method is to break off the broken bond and hammer the soft pin back out. This is marginally easier than drilling a new hole and leaves the rail looking less like a piece of Gruyère cheese. A whole day was therefore spent hitting a pointed piece of metal (a lathe centre ideally adapted to this task) as hard as possible to knock out pins in between cursing the tamper machine operator.

This seems like a good time to explain the function of a track circuit. A length of track, which can include pointwork, is isolated from the adjacent sections by insulated fishplate joints. A battery is connected, through a feed resistor, across the rails at one end and a relay is connected at the other end. If a train or vehicles move onto the section concerned then the battery is shorted out and the relay de-energises. This causes an indication in the signal box and may be used as part of the electrical interlocking. There are fifty such track circuits on the S.V.R. and they are a vital part of the electrical interlocking which supplements the mechanical locking on the lever frame itself.

Planned renewals

Turning to the planned renewals programme, the CS&TE revealed in close questioning some details of the forthcoming Bewdley North rewire and modifications to signalling (and agreed to order some new track circuit bonds, stocks having been depleted). In view of the Spagnoletti block instruments having suffered wear and tear it has been proposed to fit B.R. standard plastic ('Tri-ang') blocks and bells. During the work restricted facilities will be available, probably platforms 2 and 3 only, and a relocatable equipment building will be provided for the convenience of the signalman during the alterations to give S&T full access to the signalbox. This would probably mean only one set of points to work, the Up Home signal taken out of use, temporary block instruments and a single (Arley) token instrument. Preparatory work is likely to start this year once the boiler shop location renewal at Bridgnorth (referred to above) has been completed and will comprise running of cable trunking and finalising the design of the signalling alterations: changes to the signalling functions (including the addition of Wickenden-Crane locking) will probably be discussed at a convenient local hostelry and, in the style of the Romans, be ratified (and detail design work carried out) when sober. The work in earnest would then start in early 2000.

Meanwhile preparation is being made for the possibility that a new Carriage Shed may be built in Kidderminster Yard. A report has been produced by Hugh McQuade concerning the dispersal of the coaching stock and it may be necessary to relay the Tenbury line as far as the footpath crossing. Signalling cables will need to be traced and renewed and wide-to-gauge trap points or a wheelstop installed to protect the running lines. The possibility of a temporary carriage siding south of the bypass bridge at Bridgnorth (with a connection near the up advanced starting signal) has also been mooted. Other effects on S&T are a likely new connection to the turntable from no. 2 engine line at Kidderminster (mechanical locking modifications and splitting a track circuit) and the need to relocate some of the S&T equipment stored in the yard.

The annual 'S&T week' this year will take place in the last week of July and, by tradition, commences with a Sunday lunch on the Venturer. After an appropriate interval to sober up after the celebrations, the Department will make a concerted effort that week to get all the equipment for the Bridgnorth boiler shop location renewal in place so that it can be commissioned and tested in August. A special train will be arranged towards the end of the week to deliver this equipment, to recover redundant location cupboards and to facilitate maintenance of signals without access ladders.

The installation of the new backing signal at Bewdley North has not escaped criticism from the Locomotive Department - once more the activities of the S&T department succeed in erecting yet another encumbrance to the free movement of locomotives. To be fair the alleged criticisms appear to have focused on the late issue of the relevant signalling notice, which was issued with only hours to spare, the signal itself providing a much more visible indication to control shunting movements on the down main which should improve safety.

GTRM turned up recently at Kidderminster Junction working on the rail connection to the S.V.R. and approached the Kidderminster Station signalman, cap in hand, to borrow a rodding punch. On consulting a member of the S&T, facilities were made available to GTRM to use our rodding punch. No access charges were levied!

Routine maintenance

This is the bulk of the work of the Department but the most difficult to write about. Routine maintenance is scheduled on a regular basis: 'Greasing of Point Slide Chairs' is (almost) monthly, 'Point Oiling', 'Point Rodding Maintenance' and 'Signal Oiling and Cleaning' is three monthly, 'Point Machine Maintenance', 'FPL and Detection Testing', 'Signal Wire Run Maintenance' and 'Signal Battery Machines Maintenance' is six monthly and 'Location and Signal Contact Box Maintenance', 'Track Circuit Testing' and 'Signal Box Maintenance' is annually. Routine testing at Bewdley recently showed that all FPLs were within specification without adjustment and nothing significant was found in detection testing. The cognoscenti will appreciate that this is a significant achievement.

Detection

It is difficult in writing these notes to pitch them at the correct level. So I thought that in each set of notes I would take one item of signalling equipment and briefly describe its function. 'Detection' has been referred to several times in this article but may not mean anything to the casual reader. The levers in a signalbox are interlocked so that point levers must be in the correct position before a signal lever may be reversed to give permission for a movement to be made. However point blades could become jammed or the rodding break or bend so that the point blades did not move correctly. Detection of the precise position of switch blades is therefore provided to confirm that points, and facing point locks (FPLs) where provided, have moved correctly before the signal reading through the connection can be cleared. Such detection is normally provided for signals controlling any movement over pointwork in the facing direction and is effected either mechanically or electrically. Mechanical detection is by means of a metal slide connected by a rod to the relevant switch rail which must be in the correct position relative to the stock rail to allow a slide connected to the signal to move freely to allow the signal to be cleared. In other words the signalling system does not assume that ground equipment has responded correctly to a lever movement and it is thus robust against single failures. FPLs are provided wherever trains conveying passengers make movements over pointwork in the facing direction: the FPL holds the switch rail in place and cannot be disengaged once the train has passed the controlling signal until it has passed clear of the connection. If the detection is not properly made up for any reason, the signal lever may be reversed but the signal will remain at Danger. Routine maintenance is carried out every six months to check for correct clearances. A picture of some detection at Highley appeared in issue 120.