Train HELD

They could do with that at Mile End eastbound Central Line.

Or at least if the system **thinks** a train has been stationary for a while... I have arrived at Putney Bridge EB several times to find my train has been held somewhere... for a moment I thought I'd forgotten my train! ;D

we do not maintin or do any faults with the new type of dot matrix indicator this is done by the firm who acually installed them but i know they have been piggy backed onto the old system but for the train held indication i have no idea maybe when it sees the train stopped for any reason it shows this i.e when the signal is held at danger

I was gonna post that I've only really seen 'Held' when I've been running late, and that perhaps there was a connection - then I thought 'don't be silly, how can the TD system know I'm running late'...........your post, citysig, now has me wondering..........

Having been retired just over three years now I have no knowledge of new TD DMIs. However, it is not silly to believe that the TD system can know that you are running late although on existing technology it is not very likely.

When a train is running it drops track circuits in sequence and when it is stopped the track circuits ahead of it remain UP. The distance between any two points is a known and the length of each track circuit between two points is also a known. It is thus possible to calculate to a reasonable accuracy the minimum necessary time to cover the distance between two given points according to the maximum line speed. Because the length of a train is also a known the expected maximum time for any track to be DOWN before it picks up again can also be calculated. Thus a deviation from the expected sequential timing of DOWN tracks could be observed and used to reflect the true distance away from any given point in time.
However, there are many variables to consider such as ensuring that the correct timetable is loaded into the Programme Machine or Computer, that the signals in the route are all cleared to time, that the driver is 100% efficient, that the dwell time at a station does not vary etc etc.
Given current technology I am not sure that the necessary levels of sophistication of systems are yet in place to enable any computer to be reliable enough to take into account all possibilities at present but I expect there to be a time when such sophistication does exist.

Ever since the introduction of train radio it has been possible to determine roughly where every train is, the radio panel on the line controller desk gave such a rough indication as it indicated through which base station the the strongest radio signal was being received. In practice only the best radio signal is used to communicate from train to control, this being achieved by selection known as diversity. It could be possible to determine where a train is in relation to any given point using principles similar to radio triangulation in the open as the signal will be received at different times according to the distance in time of a train's cab from each receiving trackside base station and in the tunnels this should be easier because the train has to be between two base stations wherever it is.

So that's a couple of possibilities that could be used to control TD but AFAIK they do not yet exist and TD still relies on track circuits dropping and signals reverting to danger with the passage of trains.
Remember that the humble train ID is a problem and has been for years so much so that PTI (Positive Train Identification) had to be invented so that humans knew where trains were! Of course every train cab was unique and each cab could be identified by the unique ID of the on board Storno radio equipment. When I worked on the radio systems it was possible to interrogate the radio base stations from the control room and determine which cab was in service on any train on a tube line over the OPO(T) system although of course that unique ID meant nothing to anyone except perhaps the Car Examiner (Train Technician) and the Duty Depot Manager. We could of course also remotely determine what train number had been set up on the thumbwheels. I have no idea whether Connect Radio utilises similar facilities but I would be surprised to hear that it didn't!