Brixton bottleneck

As David says the third platform and approach line at Seven Sisters means that trains can be easily reversed at Seven Sisters (or go empty to NP depot) and the number of trains reaching the far north is less than that at Brixton - I think about every third train doesn't go beyond SS, with the NP depot service being about every 20 minutes or so.

In theory the reversing facilities at Victoria offer an opportunity to reduce the number of trains reaching Brixton - but problems with this are that the line south of Victoria is much busier than the line north of Seven Sisters, and that emptying a train at Victoria will very likely cause delay to the following southbound train.

From what I remember the Herne Hill loop idea was raised at around the same time as the Space Train idea - i.e. for implementation at the time of complete upgrading of the line, as you would probably need an extra three or four trains to allow for the loop service.

I believe there were two parts to the logic behind the loop idea. One was that a loop would allow a higher frequency (although take longer) than reversing trains in the dead-end platforms with conflicting crossover that exists at Brixton. The station at Herne Hill station would probably have been a wide island platform (similar in size to Heathrow Central) underneath the British Rail station and this would have allowed a few minutes layover for each train.

I think the other reason was that so many of the passengers at Brixton are actually to/from places further afield and simply using Brixton as an interchange with bus or rail services. Many of Brixton's passengers would be either to/from the Herne Hill area, or could more usefully interchange (especially with National Rail) at Herne Hill, thus spreading the load and reducing passenger numbers at Brixton.

As we know the upgrading is going ahead now, based pretty much on like-for-like renewal without major infrastructure changes. As far as I know the Herne Hill loop has been chucked out of the same window as the Space Train concept - I think because of the costs involved.

(edited for incompetent speeling)

Lots.



Because stephenk's concept of the flying terminus hadn't been drawn yet?

Termini such as Brixton and Walthamstow Central are the most cost effective terminus to build, which is why they were utilised on the Victoria Line. Brixton handles 28.5tph and Walthamstow 20tph in the peaks. Brixton has a platform re-occupation time of 96secs, which means with average 126sec headways then there is 30secs of operating margin & door closing time. Walthamstow has a platform re-occupation time of 90secs due to a slightly more compact crossover (and possibly different home signal position/approach gradients).

Unfortunately these types of termini become bottlenecks as the tph reaches around 30tph. Building higher capacity underground termini adds a lot for construction costs for only a few extra tph. Reversing beyond platforms helps as at Bank (DLR), but not with LUs tipping out policy. Personally I think a grade-seperated crossover on approach would be a good solution (a modern metro wouldn't build an at grade junction, so why build an at grade terminus approach!). My plans are off-line, I'll try and put them back online in the next few days.

Didn't Elephant and Castle handle 35tph before the Bakerloo was split. I know that this was "pre Moorgate", but the Elephant has two sidings beyond the platforms, so this is a similar situation to Brixton.

At a quick glance through the Bakerloo section of the library I can find 21tph scheduled on the '53 Coronation morning; 31tph scheduled in Morning Busy in 1935; 28tph scheduled just after morning busy in 1931 (this is slightly inflated by looking at departing trains that uncoupled - so arrived as one and went as two 3-cars). The theoretical minima between trains as per WTT is 1½ minutes between trains departing making a possible (and slightly interesting if it was ever scheduled) 40tph!

If time permits I'll have a look at how the schedules changed after the 1941 resignalling of Elephant. However, we've drifted somewhat off the Victoria Line.

As for the Brixton loop. We looked at this. Three drawbacks £100million cost; slow movement of trains round the loop unless you make it bigger and more expensive; two more trains required for service. There was the possibility of a station at Herne Hill but more cost and more passengers on a line already overcrowded made it a non-starter.

With regard to Brixton, platform 1 starter (VE2) will clear to white when AH, AK, BG, AL, AS, AU clear (2910'); clear to green with 373^{A - E} tracks additionally clear (4500'). Platform 2 starter (VE4) will clear to white with the crossover and flank tracks clear (BE, BC, BB, 394^C, BG) and then as VE2 (2914'); green as for the white, plus 373 ^{A - E} again (4504').

VE12 (Brixton 'Home') is 807' from VE4, 803' from VE2.

Edit: I'm a foot out on my maths - and I've just put the yellow peril away; tut, tut.

Thanks for those figures. Very interesting. Are you able to find out the distance from VE1 and VE2 that a departing train has to clear before VE12 can clear to green for an arriving train (i.e the length of the crossover)?

I would also guess that the new signalling with a 20kph speed code will allow the home signal to be closer to the crossover?



This may partially explain why the platform re-occupation time is 6secs less at Walthamstow than at Brixton.

Thanks! Stupid question, but what does TC mean?

Ye-es. I was actually thinking of Track Circuits though. In this circumstance the terms are interchangeable.