Points numbering schemes

[zbang]

Reading the thread over on the Picc line forum "Works between Acton Town and Heathrow over Christmas 2021" got me to thinking.....

Is there any rhyme or reason to how points are numbered? The diagram shows some numeric groupings but then seemingly random numbers get dropped in. Other than shifting & adding numbers as an area changes, the only other things I can come up with are interlocking and the proximity of levers (but that leaves out levers for the associated signals) and possibly the routing of rods for fully mechanical interlocking machines.

Is/was there a scheme or is this pretty much arbitrary? The same question ought to apply to signals, too. How about renumbering? Surely that's a lot of work, but must have happened at least once.

[Tom]

Traditionally the designer would lay out a lever frame with the signals for one direction at one end, the signals for the other direction at the other and, and points in the middle.

There were a few oddities, such as Earl's Court where the EB signals and points are in one group, the WB signals and points are in another, with the EB/WB connection at roughly the mid point in the frame, but by and large the rule holds true. At some sites, signals and points weren't renumbered on resignalling to keep things simple for drivers, so there are gaps in the numbering. The lowest numbered signal at Acton Town, for example, is WL4, and the highest WL116^B - despite there only being 72 levers across the two frames (the previous frame had 119).

If we use Northfields as an Example, lever one at the left hand end of the frame corresponds to the furthest left hand signal when standing at the diagram. Lever 36 at the far right is the furthest right on the diagram. (This doesn't always hold true - Oakwood is the complete opposite for example!)

When it comes to renumbering, the theory is don't! If you add a set of points or signal, you have to take your pick out of the spare levers.

At King's Cross in 2015, this is what we started with:

Lever	Function	Description
13	Spare	-
14	Signal	I.R. Homes
15	Spare	-
16	Signal	I.R. Starter
17	Signal	Starter I.R. to O.R.
18	Point	Crossover
19	Signal	Shunt O.R. to I.R.
20	Spare	-
21	Release	-
22	Release	Numbered 21x and mechanically coupled to 21.
23	Signal	O.R. Inner Home
24	Signal	O.R. Starter

We had to keep the signal numbers more or less consistent, and levers 14, 16 and 24 had to remain the same. We decided to keep 19 for the Outer Rail to Inner Rail move and 17 for the converse Inner Rail to Outer Rail, together with the crossover number (18), hence when it came to adding extra signals and points, we only had a choice of three levers to choose from. This then gave us an oddity that OJ13 was the Junction Home on the Outer Rail and 14 was used for the homes on the Inner Rail, with 15 becoming the new point lever:

Lever	Function	Description
13	Signal	O.R. Junction Home
14	Signal	I.R. Homes
15	Point	Crossover
16	Signal	I.R. Starter
17	Signal	Shunt I.R. to O.R.
18	Point	Crossover
19	Signal	Starter O.R. to I.R.
20	Signal	Intermediate signal I.R. Kings X to Euston Square - terminates move from OJ16 or OJ19.
21	Release	-
22	Release	Numbered 21x and mechanically coupled to 21.
23	Route lever	Route locking for OJ13
24	Signal	O.R. Starter

(The reasoning for the numbers starting at 13 rather than 1 is itself for historical reasons - 1 to 12 were the City Widened Lines levers.)

For similar reasons, the scissors crossover just West of Earl's Court is 5 and 23 (23 being the new lever), and South Harrow sidings (when they're complete) will have 3 and 6 as two of the point levers, simply because they are spare!

[zbang]

Thanks for the excellent explanation, but that gets me to another question (again from the Northfields thread)- the diagram shows points up to around #60 which suggests 60 "levers" (even if they're not physical parts of a frame). How many additional levers are required for signals? I imagine that many of the yard ladder* tracks may bot need them, but surely most of the crossovers and running line points would have associated signals.

*is that the correct term in the UK?

[Tom]

Ah, that's the confusing part of the diagram - it shows a mixture of depot points (which are hand operated) and those controlled from the Northfields Interlocking.

Normally, depot points are identified by three-digit numbers, but at Northfields they're only two digits. Everything to the left of 11/12 scissors and on sidings A-F are manually operated.

This photo of one of the former control room diagram panels should make it clearer as to what is controlled by the Northfields interlocking and what is manual: