No direct Chesham-Aldgate service until Dec 13th

The annual leaf-fall season is causing fairly regular signalling problems in the Chalfont and Latimer area.

In a bid to hopefully cut everyone's losses when failures occur, the pointwork is being secured each morning after the 4-car gets on to the branch, and unsecured to let it off late evening. This effectively locks it on to the branch, and in the event of signalling failure on either the main or the branch, trains can get on the move a lot sooner than if the pointwork needed to be secured in failure conditions.

It does mean, of course, that the through Chesham trains will be unable to run direct, and these trains will be diverted to Amersham.

As for connections, wherever possible these will be maintained, but there are obviously going to be one or two occasions where this is not possible. The notice is worded to take account of this eventuality, but by no means will loss of connections be the "norm."

For the Chesham customer, despite the need to have to change at Chalfont & Latimer, overall it will mean the service can run more reliably than it has on a few occasions recently.

The date of 13th December is the date when the new timetable comes in. Although the new timetable is written for the T-cup in the city area, for the likes of Amersham, Chesham and stations to the north of the line, the timetable is always renewed to take account of the end of the "leaf-fall" season.

Should the leaves and the problems they cause not be much less by 13th December, there is always the possibility that the arrangements at Chalfont will remain for a longer period.

Although I must confess, you beat me to the explanation to the 13th December - I had to go back and add that, and whilst I did that, you answered it ;D

Interesting. I understand that as part of S stock introduction the Chesham service should all through trains from about Dec 2010. S stock are block 8 car tarins and won't fit the bay road at Chalfont.
I don't recall any 'enabling works' to deal with the leaf fall track circuit reliability issue in this thread, other than I suppose the fact that an 8 car train has twice as many axles and hence a better probability of relaible operation on leaf contaminated rail. Anyone know how a reliable service will be run?

The leaves on their own don't cause these signal failures. Rather it is the interaction of the leaves and the special signalling provisions that are switched in during the leaf fall season that cause the failures. As these signalling provisions are due to be switched out again after traffic on 12th December (because the leaf fall season is officially over) then any leaves that fall after this date will not interact with the special signalling to cause this sort of failure.

There are two sorts of special provisions provided on the north end of the Met.
1) Extended Overlaps: For those that don't know an overlap is the distance beyond a red signal that must be clear before a train is allowed to approach that signal. This is to allow a margin for error in stopping the train at the signal and means that a train which does pass a signal at danger has a certain amount of additional space to stop in before there is a chance of a collision or derailment. During the leaf fall season trains are much more likely to have difficulty stopping at signals owing to the low adhesion conditions that are likely to apply. Therefore it was deemed prudent that extended overlaps be provided (thus giving a greater margin for error) on the southbound line from Amersham. The southbound line is on a falling gradient and this adds to the braking problems. Extended Overlaps are not provided on the northbound line as it is on a rising gradient which aids stopping rather than hinders it. Although it would be possible the extended overlaps are not used throughout the year as they do have an effect on line capacity.

2) Sequential Signalling. The other great problem during the leaf fall season is unreliable track circuit operation. When leaf mulch builds up on the rail head it provides an effective insulator between the rail and the train wheel. This opens up the possibility that trains will not be detected by the track circuit and thus they vanish from the signalling system. Without the special provision of sequential signalling the signal behind the train could clear even though the train is still in the section. The sequential signalling works on the idea that a train passing through an area will operate the track circuits in a particular sequence. This sequence is monitored and if it is seen correctly then the signal will clear again once all the relevant track circuits are showing clear of trains. If the sequence is not correct however, the signal will remain at danger even when all the track circuits are clear as the signalling can no longer guarantee that the train has actually left the section. It may well have done so and in this event you are left with a signal failure of the sort that has been occurring at Chalfont in recent days.

Chalfont has always been bad for this sort of problem and given the time it always seemed to take to secure up points it often seemed to me that Chalfont would benefit enormously from having 'Remote Securing' installed.

It will be interesting to see what happens when the S stock is introduced and similar failures happen at Chalfont. There will be no 4 car shuttle to "lock in" on the branch and if we are secured up for through running then I don't know what provision will be made for the Chesham passengers.

If the shuttle is to continue (or be reintroduced) post-S stock then I think LU hiring in a unit from Chiltern to operate it with would be the sensible solution. As for operation of it, then I can see two options (1) train a sufficient number of Met t/ops on the stock or (2) hire in a Chiltern crew to do it..

Option 1 would obviously involve the time in training enough operators to run the stock and would potentially lead to cancellations if there were none available at a given time.
Option 2 would involve the time required to train sufficient Chiltern crews on the route. My completely uninformed guess is that this would take less time than option 1, but might introduce union issues?

Hiring in the unit from Chiltern would sensibly include an arrangement where Chiltern maintain the unit so the costs of maintaining a non-standard train wont apply.

Another alternative would be for the shuttle to operate as a heritage service, but then you would have all the expense of non-standard stock as well as training time on either stock or route, depending who operated it.

Would it not be possible to keep two or three A stock units to run the shuttle in leaf fall season, at least their should be sufficient T/ops trained and ready to go if the need arose. Not sure what the cost difference would be maintaining old stock vs hiring trains from Chiltern, I suppose some of that would have to take into account how often a shuttle service would be required.

Surely the new trains can be set to operate to Chesham can't they. I hope thats not too much to ask

You could put an 8-car train onto the branch, secure up the relevant points at Chalfont, appoint a pilotman, and run the 8-car train up and down, only carrying passengers in one car and letting them on/off using the butterfly cocks.

That works in theory, but a lot would depend on how the signalling would cope with this method of operation -- as soon as you have to start cutting out tripcocks, you're slowing things down so much that it becomes worthless.

...and of course on the S stock you can't carry passengers in one car only, because they're walk-through.

But if you're using the butterfly cocks to open one set of doors, it doesn't matter that they can wander around the rest of the train in between whiles.

Could you operate under "one engine in steam" rules, with the branch signalling switched out?

It may be simpler just to hire in a dmu from Chiltern (or the Overground) for the leaf fall season. I know no-one who can drive them is likely to have route knowledge, but an LU pilotman could be provided.

If long-term the shuttle is to return, a three or four coach train could surely be found from somewhere on LU? D stock? 4 cars of C stock? Or perhaps tube stock - what are the platform heights like?
S stock is already planned to operate in 7-car as well as 8-car formations. How many more cars can be removed and still have working unit?

As if that's going to happen! I was lead to believe a though Chesham service was only to be run once all the S stock was in service. Has this changed? Will there be 8 car A stock running through to Chesham next year?

I wonder if a Parry People Mover has ever been suggested as a replacement for a 4-car A-stock on the Chesham Shuttle?

You're having a laugh! Least it would have transverse seating! ;D

en.wikipedia.org/wiki/Parry_People_Movers

The PPM is best suited to services with frequent stops, otherwise the flywheel has to be extremely heavy to store enough energy to get from one stop to the next. Stourbridge branch (less than a mile) - ideal. Chesham branch, much less so.

The Met should never have given up the Aylesbury service - then it would have had a diesel fleet, one of which could be used for Chesham!