What determines linespeed

[Malderon]

Hi all,

As I was enjoying my daily commute up the Arun Valley looking out of the window at the route we were travelling on (not something I've been able to do much due to darkness and/or bad weather) I was left pondering why the track is so slow.

Obviously this is the South East. Dense population etc means we don't really get an Intercity service but I was left contemplatin the Brighton Mainline in comparison which services (I believe) can travel at 90mph on. I'm not an insider but I suspect the Arun Valley is roundabout 60-75

Here's what I came up with in my head but perhaps people can elaborate or expand.

1) Signalling. I think AV line is still ancient ABS. I know this is a big constraint on capacity but I guess it also has some impact on speed.

2) Line quality - Not something I know anything about, but people always speak of high speed lines being built to a better standard. I assume ride quality comes into it.

3) Curves - The more sharp twists and turns the line has the slower the linespeed? The Arun Valley line has never struck me as particularly bendy.

4) Stopping for stations - Certainly the calling pattern on the stopping service I guess would be restrictive. However my usual service is Chichester-Barnham-Horsham-Crawley-Three Bridges - East Croydon. Certainly the Barnham - Horsham serction which is fairly long I would think there could be some speed improvements.

On a secondary note the Tattenham Corner line which I change onto is monumentally slow. seems to trickle along at about 20-30 at times although I guess the distance between stations there is a factor too. (Seems a waste that one service I regularly get is operated by an Electrostar though)

 

[swt_passenger]

According to the Sussex RUS, Arun Valley linespeeds are 65-80 mph except for the stretch through Pulborough which is 45-60mph. As you suggest the major constraint will be the AB section, but outside of that headways are also quite long, eg up to 8 mins to the north of the AB.

I have heard in the past that the notional 4 tph splitting service with the splits/joins at Horsham is the maximum possible that can be run until resignalling, due within a few years. So in both directions the fast portion will presumably be constrained by the stopper ahead.

 

[Tomnick]

Absolute Block signalling shouldn't affect linespeed (is there a maximum speed laid down for sighting a tail lamp??), but obviously the braking distance available from distant signal to home signal will be one of the factors. That's no different to the braking distances available on TCB lines though, obviously limited by signal spacing.

 

[317653]

BML gets up to 100mph at Haywards Heath

 

[Zoe]

BML gets up to 100mph at Haywards Heath

BML does not go to Haywards Heath. I expect you are talking about VTB and even then I thought trains no longer ran at 100 mph.

 

[317653]

Don;t know

 

[Malderon]

As I understand it the normal maximum line speed on parts the Brighton mainline is 90mph, but 100mph was permitted in certain circumstances for instance on "The Speed Run". I might be talking complete nonsense though.

I'm interested in general as well as in specific Arun Valley case what are the main determinants though, and how it is assessed.

 

[Bald Rick]

Think I posted something on this a while back.

There are more than 80 (eighty) factors that can determine line speed. Some are applicable only for certain things e.g. AC electrification, DC electrification, tunnels etc. I won't list them all as it will take too long!

Needless to say they all have to be checked if there is a proposal to raise line speed whereas it only takes one of them to go 'wrong' for the line speed to come down.

On the Arun Valley there are a host of factors - curvature, power supply, a number of level crossings with sighting issues, a couple of viaducts to name some of them.

On the VTB, the ruling line speed is 90 south of about Purley, and until a few years ago there was a 100 stretch from Gatwick to Balcombe, with a reduction back to 90 through Three Bridges platforms - the 100 applied to Class319s only which operated the Brighton flyers at the time. This was signed 100 as at the time it was introduced they were the only units capable of the speed on the route. Once the 377s were introduced the 319s were exclusively on trains that called at Gatwick and thus could not achieve 100mph due to their stopping patterns. The 377s on the Brighton flyers could, but because they impose significantly greater track forces than the 319s, additional track maintenance would be required. In any event the gain in journey time was somewhat less than 30 seconds, so it was decided to keep it at 90.

 

[pendolino]

On the VTB, the ruling line speed is 90 south of about Purley, and until a few years ago there was a 100 stretch from Gatwick to Balcombe, with a reduction back to 90 through Three Bridges platforms - the 100 applied to Class319s only which operated the Brighton flyers at the time. This was signed 100 as at the time it was introduced they were the only units capable of the speed on the route. Once the 377s were introduced the 319s were exclusively on trains that called at Gatwick and thus could not achieve 100mph due to their stopping patterns. The 377s on the Brighton flyers could, but because they impose significantly greater track forces than the 319s, additional track maintenance would be required. In any event the gain in journey time was somewhat less than 30 seconds, so it was decided to keep it at 90.

I don't know about FCC's 377s, but Southern's 377s are only authorised to do 90 even though they are capable of 100 (which is what it says in the cab), even on the WCML up to Milton Keynes. Apart from the special dispensation for the Brighton Speed Run, that is.

That stretch still is signed at 100 though isn't it? I'm sure there's a 100 PSR board where the 2 lines become 4 south of Three Bridges.
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On a secondary note the Tattenham Corner line which I change onto is monumentally slow. seems to trickle along at about 20-30 at times although I guess the distance between stations there is a factor too. (Seems a waste that one service I regularly get is operated by an Electrostar though)

Tell me about it! Quite a difficult route to learn at first because the speed changes so many times on the Tattenham branch; and sometimes the speed on the up is different to that on the down. Generally the low speeds are largely down to track curvature as far as I'm aware - especially at somewhere like Smitham (or Coulsdon Town as it's soon to be known).

Chipstead Valley Viaduct (between Chipstead and Kingswood) has a 20mph PSR on the up but 30 on the down, as it was found that trains on the down trying to accelerate up the gradient towards Kingswood were slipping to a complete standstill during leaf fall. The speed was increased to give us a better run up!

 

[LE Greys]

Power supply, I'd imagine, is a major one. Anyone who's travelled in a 319 or 377 on both overhead and 3rd rail will know that units tend to be sluggish on d.c. Presumably, this is because of voltage drop and a restriction in the number of amps you can get down a live rail. Also, the Arun Valley hasn't been upgraded since electrification, when the speed was fairly high in comparison with the rest of the network (mostly steam then) so that might be a contributing factor.

 

[pendolino]

I don't think power supply influences the maximum permissible speed on a line, but it certainly does affect train performance (a 12 car 377 is noticeably sluggish compared to a 4 car), and the CLI (Current Limit Index) on DC lines does determine the maximum length of train. As an example, work is currently under way to allow longer trains to run to East Grinstead, but it's not just lengthening platforms that's involved. The power supply infrastructure (much of which dates from the 1930s apparently) is also being upgraded, if it wasn't, the longer trains would frequently trip the circuit breakers at the substations.

There have also been cases where, due to substation faults, drivers have been instructed not to use weak field (full power). No accompanying reduction in linespeed, but it has meant that acceleration to linespeed has been slower.

(That was a bit off topic....)

 

[Bald Rick]

I don't think power supply influences the maximum permissible speed on a line,

I'm afraid it does. Assuming the same number of trains in the system at peak load before and after a speed increase, then higher speed = more power. The additional power is needed both to accelerate the train to the higher speed and to keep it there. Think about how fuel consumption for cars increases at 80mph vs 60mph

 

[Anonywave]

BML does not go to Haywards Heath. I expect you are talking about VTB and even then I thought trains no longer ran at 100 mph.

huh? BML (Brighton Main Line) is the common term for the Victoria (or London Bridge) to Brighton Line.

 

[OxtedL]

Just thinking that...

...ELR through Haywards Heath is VTB3, BML refers to other stuff (specifically SWML out of Waterloo, wierdly)

I knew what was meant though, as should non pedants! BML is a common abreviation for Brighton main line.

 

[Zoe]

BML (Brighton Main Line) is the common term for the Victoria (or London Bridge) to Brighton Line.

It may well be commonly referred to as the Brighton Main Line but BML officially stands for Bournemouth Main Line.

 

[OxtedL]

Ah, all becomes clear.

Sorry for possibly insinuating pedantry btw.

 

[Ivo]

I've never heard of the Bournemouth Main Line; I've always thought that it was part of an (extended) SWML?

 

[Zoe]

BML is used for the entire route from Waterloo to Weymouth. It is commonly referred to as SWML but SWM is actually South Wales Main Line.

 

[TDK]

BML does not go to Haywards Heath. I expect you are talking about VTB and even then I thought trains no longer ran at 100 mph.

I used to do 100mph through Gatwick on the Brighton Main Line in a 319, took a while getting there but on a good unit there was not a problem, I don't know whether it is still permitted though.

 

[ChrisCooper]

On the VTB, the ruling line speed is 90 south of about Purley, and until a few years ago there was a 100 stretch from Gatwick to Balcombe, with a reduction back to 90 through Three Bridges platforms - the 100 applied to Class319s only which operated the Brighton flyers at the time. This was signed 100 as at the time it was introduced they were the only units capable of the speed on the route. Once the 377s were introduced the 319s were exclusively on trains that called at Gatwick and thus could not achieve 100mph due to their stopping patterns. The 377s on the Brighton flyers could, but because they impose significantly greater track forces than the 319s, additional track maintenance would be required. In any event the gain in journey time was somewhat less than 30 seconds, so it was decided to keep it at 90.

Seriously, the 377s have a significantly greater track force than the 319s? I'd have though a 50t 319 motor coach would be worse than the slightly lighter 377 DMs.

Absolute block has no effect on maximum speed. There are a few 90mph AB sections around (Norwich to Ely for a start), and previously there were plenty of 100mph AB sections (pre-HST ECML, pre Leicester resignalling MML etc). 100mph signal spacing is fine for up to 125mph with trains capable of exceeding 100mph, as they all have superior braking (electopnumatic disks, rheostatic or both) so can stop from 125mph or 110mph (in the case of the 87s and 90s) in the same distance other trains stop from 100mph.

 

[Hydro]

I've never heard of the Bournemouth Main Line; I've always thought that it was part of an (extended) SWML?

BML is what is known as an Engineers Line Reference, the three letter code (plus one number if divided into more than one section) that identifies the line to the infrastructure people. BML is Waterloo-Weymouth, and has three sections; BML1 - Waterloo to Northam, BML2 Northam to Moreton (IIRC) and BML3, Moreton - Weymouth.

 

[Ploughman]

ELRs can be seen on most Quail maps.