SSR Resignalling

I understand what you are saying - its not an aspect that I had considered.

Can outer rail Circles reverse in the Met platforms (if available?!)

Moorgate is a very poor alternative, although the advent of the Elizabeth line's connection with this station does mitigate some of the draw backs.

The Baker Street plat 3 crossover was promised to return when the whole layout was renewed, but that was subsequently dropped!

Does this mean that an S8, entering the outer rail platform in error, can now continue to Tower Hill to reverse? A long while since that station saw an 8-car train!

Yes they can. Additionally since resignalling all four platforms at Moorgate can be used to reverse the service - previously Platform 2 was Westbound/Inner Rail only, there is now an EB move into it.

No, there are no booked "rusty rail" trips for S8's to either Tower Hill or Whitechapel.

Similarly, there's a number of moves around Edgware Road, High Street Kensington and Gloucester Road that Circle and Hammersmith & City drivers are fully trained on but District drivers are not.

If such moves are required, a "pilot" must be sought to guide the driver. A pilot must be familiar with the moves concerned.

Only when there are non-communicating trains running around.

It is worth noting that the existing layout cannot be replicated with flat bottom rail components and these are now required by the LU Engineering Standards. Standard components tend to offer greater reliability. It was understood at the outset of the Sub-Surface Upgrade in the early 2000s that this move would eventually be lost when the junction work at Aldgate was renewed and the new (and fairly expensive) facility at Tower Hill was partly intended to provide a replacement. Despite this, options to include the reversal from platform 1 have been reconsidered recently, but would have added very significantly to the cost and complexity of the renewal...

Would be a waste of money if I'm being honest, sections of the Underground still use perfectly good bullhead rail. If a section is re-railed then it'll get replaced with flatbottom rail. Although in the past 10 years sections have been re-railed with bullhead rail. Eg, Stratford in 2010 if I remember correctly?

I am fairly sure that Wimbledon Branch is three-aspect signalling between East Putney and Wimbledon so couldn't that be upgraded to four aspects by replacing signal heads and installing extra signals halfway between the current signals? Also does the differential speed limit affect capacity with LUL trains allowed to do 45mph where as NR trains are limited to 30mph?

If only it were that simple.

Whilst it is a fact that more signals = more capacity, there is a limit as to how many signals you can bung in.

Signals are positioned according to a number of factors, such as the fastest speed of trains, the weight of trains, the braking capacity of trains, gradients, etc.

Four aspect signals would be pointless as the maximum line speed is 45mph; there is absolutely no need to know that you're three sections ahead clear at that speed!

At one end of the National Rail signalled area you've got a 30mph LU signalled section and at the other end a 10mph four platform terminus with a number of conflicting moves. I can't really see what's to be gained over such a short section with those constraints at either end of it.

And does the 30mph speed limit for South Western Trains restrict capacity? Absolutely! And more than that, they create havoc when they crawl into Wimbledon Park depot at 5 mph! It's also worth bearing in mind the reason why they are restricted 30mph; their braking capacity is much less effective compared to LU stock - in other words it takes them a far greater distance to come to a stop. That has to be factored into the signalling system!

I'm not so sure Colin. Once CBTC comes in on the branch as far as East Putney the traditional constraints associated with the signalling capacity are (in theory) gone, so it might well be the fixed-block section that becomes the pinch point.

Theoretically, if you go from 3 to 4 aspect but the line speed and braking distances don't change, this is what you do - simply halve the distance between signals and stick another in at the half way mark. The critical thing is making sure the braking distance from the first cautionary aspect to the red remains the same - I believe this has been done on some of the LTS section where it runs parallel to the District.

However, it does come with significant initial and whole-life cost and with all such things there is a law of diminishing returns - especially on a signalling system that is over 30 years old and getting near to the end of its design life.

I note the branch is heavily restricted in what national rail stock can run along it, but I would question if the braking capacity of EMU stock is much less effective - they were traditionally similar to LU in terms of braking systems and on four aspect sections where the signal spacing had to take into account a mix of traction and rolling stock, it was suggested that drivers of EMUs did not require the full braking distance from a double yellow to a red and were still able to stop comfortably from a single yellow (though the Human factors concerns about this practice are for another discussion!).

I'll throw my 5 eggs in here an suggest any capacity increase on NR metals in the SW area will await Waterloo area ETCS and no-one will upgrade or resignal anything before then.

I can't remember now where we are with these things, but it is something like NR ETCS on GN main line routes from Kings Cross is the first main line ETCS area scheme, second GW from Paddington, and third SW from Waterloo***; yes all this is yet a long way off - but it real terms but I'll suggest we'll get to that point in time way before anyone finds money to tear up signals and track circuits just to jump from 3 to 4 aspect signals along here.

& before someone mentions these things - yes I am aware Thameslink core and ex-Northern City have ETCS in some shape or form - I am talking about about main line area schemes not pockets where local specific circumstances take advantage of it.

There will be a need to interface LU CBTC with NR ETCS but that is manageable, including it's implementation on S-stock+++. Outside my domain but I can think of at least 2 ways it could be done.

Also, one other thing to remember is these days at least as far as main line trains are concerned, double yellow on a 4 aspect signal is much more restrictive than it used to be.

The original concept behind the YY aspect was, in simplified terms, freight trains (then majority unfitted) commence braking but passenger trains commence braking at Y.

In these days of main line defensive driving, YY is now commence braking for all trains; in turn has a negative knock on effect of throttling capacity. The theoretical capacity jump from 3 to 4 aspect signalling is much diminished.

Not braking at YY is pretty well standard checks carried out in random audits of main line train OTMR and is regarded as a serious issue almost possibly even treated equal to a SPAD.

How a LU train would be managed with in this respect I can not comment on.


*** one, of several, reasons behind the incumbent SW TOC went / going for total demolition / displacement of the mixed up 455 456 458 707 suburban EMU fleet and replace all with one type (701) is those 701 are all ETCS ready hence easier and cheaper to activate when needed. ((The issues with actually getting 701s into service are irrelevant.)).

+++ and once you have done that, it more or less hands you the solution to kill the retained Amersham Harrow clockwork signalling for Chilterns as by the time we get that far, Chilterns or whoever they are by then will almost certainly have had the fleet replaced by new whatevers sourced from whoever that will be ETCS ready.

Well, yes, I had not thought of it that way, it certainly opens up alternatives, wherever / whatever train protection is in use.

Maybe the differential speed limit is because the SWR trains pass through stations without stopping?