The Information on dot matrix displays is indeed derived from the TDs. I think in another post you said you were at Rickmansworth (training?) The number part of the Met TD refers to the stopping pattern. They have been used since the current sub-surface TD system was installed in the early 1960s and thus predate the SCC.
As far as I know the Met is unique in having numbers as part of its TD.
Within the TD equipment each TD is represented as a 5 bit code known as ABCDE code giving 31 possible different descriptions. As an example AM5 might be ABD while AM2 might be ABE. At each location where it is necessary to display the TD, whether on the platform or in the signal cabin, the ABCDE code is fed into an annunciator combinator which has 5 inputs (ABCDE) and 31 outputs, one for each combination of ABCDE. These 31 outputs are connected to the display device such that the correct description is displayed. Several outputs may be looped together if it is desired that they produce the same display.
For example, if memory serves me correctly from when I worked Ricky box, on the northbound only the Letter parts of the TD are shown. So for Amersham AM is shown regardless of whether the TD is AM0, AM2 or AM5. In other words the three different outputs produced by these different codes are looped together and connected to the input of the display device which causes it to show Amersham.
On the southbound at Ricky however both the letter and number part are shown and you will notice that this is displayed on two display units side by side. In this case the ABCDE code is fed to two annunciator combinators in parallel. One provides the output for the letter part of the display as for the northbound, while the other provides the output for the number part and all TDs with the same number will be looped together.
This is a quick description of the 1960s system and leaves a lot out. The only part of the Met where the platform describers are driven directly bu this system as described is at Harrow, although this system would have controlled all th platform describers south of there prior to the opening of the SCC.
The Dot Matrix displays in the area controlled by the SCC take their information from the trip data for each train contained in the computer system. This trip data contains a TD element and the same TDs are used more or less. e.g. the BK0 TD becomes BKM0 to the SCC, A0 become ALD0.
For your information the stopping patterns relating to each number, as originally specified are:
(Shown Northbound from Baker St but apply equally southbound)
0 Finchley Road, Wembley then all stations
1 Finchley Road, Wembley, Harrow then all stations
2 Finchley Road, Harrow then all stations
3 Finchley Road, North Harrow then all stations
4 Finchley Road, Rayners Lane then all stations
5 Finchley Road, Harrow, Moor Park then all stations
6 Finchley Road, Moor Park then all stations
7 Finchley Road, Wembley, Harrow, Moor Park then all stations
A Finchley Road then all stations via Jubilee Line
E Empty.
Note that because the system is limited to 31 different TDs it was not possible to provide all possible combinations of destination and stopping pattern. For example Uxbridge TDs provided are UXA, UX0 and UX4. There is no UX2 TD hence while it is many years since fast Uxbridge trains ran non stop to Rayners Lane the semi fast Uxbridge trains, first stop Harrow, continued to use the UX4 description until September this year when semi fast Uxbridges were abolished as well. Similarly while the off peak northbound service to Amersham correctly uses the AM7 description the southbound service has to use the BK6 description, which had previously fallen into disuse as no BK7 description was provided. In both cases the platform describers were modified accordingly.
The other major change over the years has been the provision of multiple descriptions equivalent to a BK0.
Specifically BK3, Bk4, BK8 & BK9 now all mean the same as BK0. This modification was indeed provided for the benefit of the SCC. You will notice that in the timetable there is never an occasion where any of these descriptions is used by two consecutive trains. The reason for this is to make it easy for the computer to correctly identify the trains as they come onto the SCC system at Wembley. At the time the SCC opened there was no off peak service to Aldgate and it was expected that the vast majority of trains arriving at Wembley from Harrow would be BK0 trains and so the multiple TDs were provided so the computer could tell the trains apart even if they are not running in the correct order. In general Baker Street trains running from Watford alternate between BK3 and BK4, while those from Uxbridge or those that originate at Harrow alternate between BK8 and BK9. The TD system does not cover the Uxbridge branch so all trains from there have their TD set up by the Harrow signalman. This is the reason the Uxbridge branch is still without any form of platform train describer.
