(14-04-2010, 02:20 PM)sugavanam nagarajan Wrote: Dear Sir,
1.Please explain the working principle of train describer:
How unique train number is identified by TD?
-Sugavanam
The train describer was originally a means of communication between adjacent signalboxes. When signalling was semaphore and absolute block, trains were offered from one signalbox to another by bell code: "Is line clear for an express passenger train?" or "Is line clear for a unfitted freight train?". However sometimes it was important for routing information also to be transferred, so there were some site specific codes to tell the next signaller which way the next train was to be routed at the diverging junction under their control.
At the busy station, as technology developed, this information was later transmitted from the adjacent box by electro-mechanical means; often a clock face with a pointer that rotated around by clockwork to indicate one of many descriptions to act as a visual reminder of the type and destination of the train which was approaching the area. These were the first "Train Describers".
Then as signalboxes grew to control a larger area (generally some form of power operation) there could be multiple trains on the same line within that signalbox control area and therefore some aid to the signaller was needed to remind them which train was which. Hence the train desciber developed into a "stepping TD" so that a code was entered (or indeed actually transmitted before the train's arrival from the previous signalbox) to a "berth" at the fringe of the signalbox area.
When the train occupied the first track circuit, the description steps from a "train approaching" display into another display associated with the first stop signal. Then when the train arrives at and then passes that signal, the TD then jumps to the following signal (if there are diverging points then the one is selected according to the point lie) and so on until the end of the signalbox area. Hence the description moves around a panle (or VDU screen) mimicking the movement of the real train on the track circuits.
The cform of the code used in the UK to identify each train is generally of the form of 4 characters:"number", "letter", "number", "number". The first says something about the "class" of the train; for example "1" is the (historically) the most important being the fastest (i.e express passenger train) and "9" for those freight train that had to go slowly because of the type of wagons used and the lack of brakes; in recent years this distinction has changed and "class 9" has been reallocated to be Eurostar and the NR track access rules are not based on train speed so the historic situation is no longer true but it gives the idea.
The letter gives a clue to the destination of the train, but this does vary around the country; on the Western "A" was for London Paddington to the West of England (=Devon and Cornwall to Penzance), "B" for Bristol via Bath and "C" for Cardiff and onwarsds into South Wales etc.
The last two digits was a sequential number identifying the train, but with even numbers being used for the one direction and even numbers for the opposite; so the train following 1A36 would probably be 1A38; it may not have exactly the same calling pattern but it would be following the same route towards final destination.
4 charater TDs can't always be 100% unique witrhin the working day, but obviously it is intended to avoid two trains running simultaneously with the same TD identification number since that would lead to confusion. The Europeans tendd to use a 6 digit (all number) type description- but I don't know the details of allocation but I suspect that these are unique to every train on the network but without the "intrinsic meaning" of the UK system.
TDs are what are used to give the correct information to the passenger and railway staff re train running; nowadays this TD information is not only available in signalboxes but to many rail staff and indeed indirectly to the public whether they are at stations looking at train departure / arrival screens or indeed using the internet to check that their train is running on time and has not been heavily delayed or even cancelled. This is achieved by a system called SMART that interfaces the TDs from all the signalboxes to another system called TRUST. See for example my local station train departure board info which is updated nearly in real time.
TDs are also important when there is CSR= Cab Secure Radio. If the signaller wants to talk to a specific train driver then they enter the relevant TD into the system. This identification code is used to determine where the relevant train is (i.e which signal it is next approaching) and this in turn is used to determince what radio channel was allocated to that train when it enteed the area covered by a rado base station. This enables the system to route the call to the cab of the correct train.
Quote:2.Is it SSI only having SPAD Alarm or RRI also?Good question; generally it is provided on IECC and other VDU based control systems and yes these generally are equipped with SSI or some other computer based ignalling system. I believe that the current standards require SPAD alarms on panels and RRI, although I can't recall ever having seen one implemented on RRI (but suspect that there may be one somewhere). Certainly at my local siganlbox (1960s RRI and panel) there is no SPAD alarm as such, but changes in recent years has implemented aspect reversion following a SPAD on certain signals.
Explain how SPAD Alarm system is working?
Basically the system looks for a sequence of track circuit occupancy (e.g. berth TC occupied then stick track occupied) that suggests that a train has passed the signal at the time when the signal not showing a proceed aspect. The track indication on the VDU screen is given a yellow background and a distinctive audible alarm is given until it is acknowledged and also a text message appears which states at which signal a SPAD has been detected. The signaller than needs to take the appropriate action to address the risk.
These have only been provided for the last 10 years approximately (following accidents such as Southall and Ladbroke Grove); practically no new panels have been commissioned during that time. Basingstoke doesspring to mind and I don't think there is anything on the panel fascia itself; I suspect that some variant must be incorporated into the overall TD system so I guess there is a specific monitor that lists any SPAD alarms possibly with a representation of the layout to be able to mark the location.
Indeed for a long time a TD had the feature of an NDA = Non Described Alarm, which is really the forerunner of the SPAD alarm. If the track beyond a signal becomes occupied, the TD would normally step the description it held for that signal into the following one; if there is no TD at that time for it to step it generates a special one and sounds an alarm to the signaller to draw attention to the situation.
Historically this I suppose was a sort of development of the TRA = Train Running Away buzzer. When a mechanical box was extended to include a colour light signal to provide an additional block section (or more usually to replace the adjacent signalbox so money could be saved by closing it), the "Intermediate Block Home" signal was always remote from the signalbox and operated by track circuits. Unlike the other section signals ithe signaller was much less aware of happenings in its vicinity. The TRA was provided so that if it were passed at danger the signaller got a buzzer and could then at least warn the adjacent signalbox by sending an emergency message on the block bell; it had to be assumed that there were now two trains within the block section and a collision might result........
PJW