Railway signalling discussion

Permissive Tablet Working on Hertford Loop

British signalling of the past (UK, excepting Northern Ireland)

Permissive Tablet Working on Hertford Loop

Unread postby Mike Hodgson » Fri Feb 17, 2017 11:37 pm

I recently got on ebay an interesting set of GNR regulations applicable to the then single line between Langley and Cuffley, dated March 1918. These would have been valid from opening until June 1924 when the line was doubled. The regulations cover both absolute and permissive operation using Tyers No 5 tablets. I should point out that there was no passenger service during this era and intermediate stations did not open until the line was doubled. This includes the current Hertford North which replaced another station of the same name which had been terminus of the branch from Hatfield/Welwyn.

There are a number of features of these rules which I think people will find interesting.
There are several bell codes peculiar to this instrument, including some codes which have very different meanings under other contemporary regulations, the following are of interest, and it would be interesting to know how these codes compare to those used on the Reedham-Breydon instruments (the only other UK section I've heard of with them)

5-3 Release not effected
4-2 Commencement of Permissive System
1-2 Adjust Instrument (not needed as a branch ILC code – no branches)
1-3 Instrument set for Permissive System (not branch ILC code – no branches)
2-4 Termination of Permissive System
3-1-3 Instrument restored to Normal
2-2-4 Cancelling Adjust Instrument
4-4-4-4 Extract Tablets for Transfer – rebalancing done by the signalmen themselves, extracting as though for trains by switching to permissive, until exchange of the following bell code
4-3 Extraction of Tablets Complete. Tablets were placed in a special box and conveyed by/via the station master to the other end of the section where they were restored to the instrument by normal procedures for termination of permissive working. If no suitable train was available, the tablets would be worked via Finsbury Park.
1-2-1 ILC for Rail Motor Cars running Empty
4 was available for (diverted) expresses and 3-1 for ordinary passenger trains (AB only of course), but 3-1 also included “Excursion & Special Passenger Trains stopping at Intermediate stations”, although there were no intermediate stations in the section at that date!

The Tablet carrier was important:
for trains signalled under AB, an absolute carrier was used, the hoop having a circular handle
for trains signalled permissively, the permissive carrier had a triangular handle
if an appropriate carrier was not available, no carrier was used and a written notice given to the driver instead
the driver was required to observe the carrier as well as the tablet

The tablet number had to be recorded in the train register.

It was possible to cancel a train and restore the tablet to these instruments.

Ballast Trains requiring to work in Section had to be signalled under AB, special stoppages in section also had to be confined to trains with a tablet in an Absolute carrier

Regulation 9 (platelayers trolleys going through tunnels) was “not applicable to this line” despite the section including Ponsbourne Tunnel, the longest in the Eastern Region at 2684 yds, also Molewood Tunnel 364 yards.

This was a very long section for permissive working. No train from Langley was to follow another for 10 mins, and the same interval from Cuffley and additionally a plunger at the north end of the tunnel had to be operated by the trainmen. It would not take a great difference in speed for a following train to catch up in a section 15 miles long.

As very few can be familiar with No 5 instruments, it may be helpful if I describe my understanding* of its operation. An illustration (for the GER Reedham Junction – Breydon Junction section) can be found here :

* correction of any misunderstandings welcome!

There are two plungers marked Bell and Switch

There is another plunger at the front which must be depressed after each manipulation of the big wheel on the top of the instrument.

The indicator (a small lever just above the Tyers patent label and below the galvo) has a normal position of Line Blocked
after ILC has been accepted under Reg 4 or 5 both signalmen must move this indicator to Line Open
it must be returned to Line Blocked when Train out of Section is sent (Absolute Block) or when the whole single line is clear (permissive)
The needle should deflect on each depression of the bell plunger; if it does not do so, it is assumed there is a defect.
Whenever the bell plunger is held in for a release, this should last about 3 seconds; if the signalman requiring a release does not get it, the special bell signal Release not Effected (5-3) is sent, and the signalman acknowledging this will hold in again on his last beat.

When working under AB
signalman B holds down on the last beat as with other single line instruments,
A holds down the plunger marked Switch and turns the upper wheel in the middle of the instrument anti-clockwise and lifts the lid at the front of the instrument to take out a tablet.

On arrival
B lifts his lid, inserts the tablet and sends TooS
When A acknowledges he holds in his Bell plunger on the last beat.
B then holds in his Switch plunger and turns his upper wheel clockwise.

To start working permissively from A to B, the instrument must first be adjusted. This requires a tablet be extracted as though for a train in the other direction, but instead of giving it to a driver, B inserts it into his slide (which resembles the slide on a No 6).
A sends the special signal Commencement of Permissive Working (4-2).
B acknowledges
A holds in his Bell plunger
B holds in his Switch plunger and turns his upper wheel anti-clockwise, lifts his lid and extracts tablet
B depresses his upper wheel and pulls out his slide, inserting the tablet. The slide is then pushed back in and held in while the upper wheel is fully depressed. Releasing pressure on the slide locks the wheel down.
B then sends the signal Instrument set for Permissive Working (1-3)

Before each train is offered under Permissive Block,
the special signal Adjust Instrument (1-2) is exchanged
A holds down his bell plunger
B holds in his switch plunger and turns his wheel clockwise

The first permissive train is then offered by A and accepted in the same way as for AB.
ILC for following trains (if the first is still in section) are accepted by Line Occupied (3-4) instead of repetition. When A acknowledges that, B holds down his bell plunger and A extracts a tablet the same way as he does for AB.

On arrival,
B restores the tablet to his hopper and turns the lower of the two wheels clockwise and sends TooS
A acknowledges, but (unlike under AB) there is no need for B to press his Switch plunger

Once TooS is sent for the last train and all the tablets are back in the instruments
A sends Termination of Permissive Working (2-4)
B acknowledges, holds down his upper wheel, presses on his slide, lifts the upper wheel to its full extent and slightly depresses it again
This unlocks the slide. The tablet is withdrawn and the slide pushed back in.
B then lifts the lid and restores the tablet to the hopper and sends Instrument Restored to Normal 3-1-3
A acknowledges, holding in on the last beat
B presses his Switch Plunger and turns his upper wheel clockwise.
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Mike Hodgson
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