Indicator Boards

The boards are mostly driven from the underlying train describer system. Train descriptions (TD) are associated with a train throughout its journey and is transmitted forward to the next location as the train progresses. In the case of the boards at Harrow the description is displayed as soon as the associated TD is transmitted forward to the platform location within the TD system. This happens when the signals are cleared for the train to enter the platform and the train has passed a specific point. When the train leaves the platform the TD is automatically transmitted forward and the indicator board goes blank. The signalman at Harrow has the ability to cancel the received TD and set up a different TD if required. In the case of trains from the Uxbridge branch the TD always has to be set up manually in this way as the TD system does not include the Uxbridge branch. Down children trains also have to have their TD set up manually.
For northbound trains the TD is transmitted forward to the platform as soon as they leave Wembley Park, provided the signals at Harrow have also been cleared to signal the train into a platform. The sole exception to this is an all stations train signalled into platform three. In this case one of the signals in the route is approach controlled so even though the route is set up long before the train gets there the signal does not clear until the train is closely approaching Harrow, and it is only then that the TD will transmit forward to the platform.
For southbound trains the TD will transmit forward to platform 6 as the train passes over Harrow North junction, provided of course that the signals are clear for it. If however the train is to be routed to platform 5, there is again an approach controlled signal and the TD only transmits forward when this signal clears. This happens just before the train reaches the road bridge north of the station.

Just to add to this these describers were first installed in the 1960's and methodology described would seem to be that employed most of this time. However, they have been modified to add Ruislip as a destination when the turnback siding which links to West Ruislip depot was built and they have been broken by vandals on odd occasions and the glass fronts replaced. Aylesbury Vale Parkway has never been added to the describer of platform 1 used for the Chiltern service.

On a similar note an article in the local Harrow Observer posed the question as to why there were no indicator boards at Rayners Lane. The answer was that there will be once re-signalling is finished. Perhaps the indicators at Harrow will be modernised at that time.

I hope not! The lightboxes are ideal for the job. Standard LU dot matrix indicators are IMO ill-suited for Harrow on the Hill.

Yes it comes from the same source. There is a train movements application on our internal network that draws in info from control systems and other locations such as signal cabins. This is then passed onwards to the public feed that app developers and the TfL web site use. The accuracy is only as good as the underlying source and I recall that there are quite a few gaps on the extremities of the Met and east end of the District where some assumptions about movements have to be made.