Why is the linespeed 85 mph?

[notadriver]

In my area there are a number of places where the speed limit is 85 mph (on straight track) Why can't they just make it 90 mph? I'm just wondering if there is some historical or engineering reason behind the 85 mph speed limit if anyone knows.

 

[hairyhandedfool]

Presumably they are 85 mph because 90mph is too fast :roll:

I imagine there is a fairly large variety of things that can affect speed restrictions, but very few will be historical.

 

[Trog]

Signal spacing maybe?

 

[ainsworth74]

I suspect unless you tell us which bit of track you're talking about no one will be able to give you answer beyond vague generalities

 

[Carlisle]

Older drivers have told me in the past places like shap and Lancaster have had their speeds raised by 5 or 10 mph and it has then been returned to the lower limit so i guess there are probably quite a few cases like this nationwide for various reasons

 

[RPM]

I thought railway linespeeds were always the theoretical maximum safe speed rounded down to the nearest 5mph? That's what I was taught anyway.

 

[LNW-GW Joint]

The North Wales Main Line was supposed to have been upgraded from 75mph to 90mph from Chester to Llandudno Jn around 2000 (paid for by the then Welsh Assembly).
In the end, 90mph was only achieved in a few places over short distances, and there were some longer stretches of 85mph. 75mph was retained through stations.
Signal sighting (old semaphores, mostly) came into it, and I think there is a more restrictive policy on level crossings at 90mph.
One of the stretches kept at 75mph was the 5 miles Saltney-Connah's Quay.
However even after this stretch was resignalled a few years later, the line speed hasn't changed, despite being dead straight and level and a former 4-track route.
There are now plans for resignalling the whole line west of Connah's Quay, so hopefully a consistent 90mph can be achieved.

 

[edwin_m]

There are many factors influencing line speed, including:
- Curves, cant and transitions
- Signal spacing
- Signal sighting
- Level crossing strike-ins
- Sightlines for passive level crossings
- Track quality and maintenance category
- Ground conditions

Probably a few more too. Sometimes I suspect the permitted speed was set way back in the mists of time, the person that did so has long since retired and if there ever were any records they have been lost!

Hence any proposal to increase the line speed has to investigate all these issues and decide whether any of them make it unfeasible or unaffordable. This is quite a complicated exercise and will only be done if somebody pays for it, hence only if there is seen to be a good reason to reduce journey times. It can be easier to increase line speed if the infrastructure is due for renewal, for example at a resignalling it costs relatively little extra to put the new signals in at a wider spacing instead of the existing positions, but it would cost a lot more to start moving existing signals around.

Even Network Rail's own long term planners tend to duck this one, sometimes preferring to work out the financial benefit from reduced journey time and say if that many minutes are achievable for that much money then it is worth doing.

 

[Carlisle]

I thought railway linespeeds were always the theoretical maximum safe speed rounded down to the nearest 5mph? That's what I was taught anyway.

That sounds like a pretty robust theory ,although i guess there could be a hundred plus different reasons why a certain speed exists in a certain location

 

[455driver]

We have a 70 line where there is a (gradual) curve where the line speed dropped to 50 for no apparent reason, a few years ago it was raised to 70 because nobody had a clue why the lower limit was there, so we now have 3x 70 boards on a row, one where the 70 starts, the second where the old 50 started and the 3rd where the linespeed went back to 70.

A simple reason for yours might be 'because it always has been' and nobody has got the backbone to change it to the 90.

 

[swt_passenger]

Heading UP past Northam depot the line speed is shown as 80/85 (HST). (Where HST refers to XC Voyagers.)

That tells me that they definitely do work out the speed limit for different stock characteristics to the nearest 5 mph increment, and simply rounding up to 90 from 85, which is what the OP seems to be suggesting, would not be done.

 

[The Planner]

That one will be down to signal spacing as HST graded stock can use a different braking appendix speed in the railway standards. If i recall, Appendix A is pretty much the standard braking curve for freight which everything will fit into, B is for passenger and C is for passenger stock with the ability to brake at a certain m/s²

 

[yorkie]

Heading UP past Northam depot the line speed is shown as 80/85 (HST). (Where HST refers to XC Voyagers.)

'HST' also includes Class 91s & Mk4 coaches and certain classes of multiple units which have enhanced braking characteristics. As you say Voyagers are included in this.

 

[Carlisle]

Heading UP past Northam depot the line speed is shown as 80/85 (HST). (Where HST refers to XC Voyagers.)

That tells me that they definitely do work out the speed limit for different stock characteristics to the nearest 5 mph increment, and simply rounding up to 90 from 85, which is what the OP seems to be suggesting, would not be done.

You also have Berriton tunnel on the Portsmouth direct line ,the old mk1 units used to fly through at 70 now everything seems to have to slow down to 40 i presume for clearance purposes

 

[A-driver]

I know there are busier areas where line speed is lower than it needs to be mainly for keeping maintenance costs down due to traffic volume aswell as due to signal spacing. The London bridge-east Croydon line for example is 70 from new cross on the fasts but the 'record breaking' London to Brighton 377 run took it at 80 along with other sections of the BML which it was allowed to 'speed' on. The reason this was allowed was because it was a one off run so wouldn't impact maintenance requirements and also they gave it a clear run so it wouldn't need to worry about signal spacing as there was no other traffic I front of it.

 

[swt_passenger]

'HST' also includes Class 91s & Mk4 coaches and certain classes of multiple units which have enhanced braking characteristics. As you say Voyagers are included in this.

Sure - I was really just pre-empting someone saying that HSTs themselves don't run in that area...

 

[Class172]

I'm sure I read in some document that the XC line as it by-passes Worcester could be 100mph+, but the number of LCs in place prohibits this so it remains as 90 (I think).

 

[JB25]

I know there are busier areas where line speed is lower than it needs to be mainly for keeping maintenance costs down due to traffic volume aswell as due to signal spacing. The London bridge-east Croydon line for example is 70 from new cross on the fasts but the 'record breaking' London to Brighton 377 run took it at 80 along with other sections of the BML which it was allowed to 'speed' on. The reason this was allowed was because it was a one off run so wouldn't impact maintenance requirements and also they gave it a clear run so it wouldn't need to worry about signal spacing as there was no other traffic I front of it.

Not worked out why it is only 70mph. In a 377 you could easily get the speed down from 80 or 90 in time, and 455s would struggle to get much above 70-75 along there anyway.

 

[Tiny Tim]

I was about to say that 85mph had a rounder, metric equivalent, but it's 136.79kph, so no.

 

[asylumxl]

A simple reason for yours might be 'because it always has been' and nobody has got the backbone to change it to the 90.

This is the reason I would go for.

Seems to be the way with the UK railways. I guess if there is no real advantage then I guess the attitude is "If it ain't broke, don't fix it".

 

[Old Hill Bank]

I never have understood why the class 33s were built as 85 mph locos, I can only assume this was specified to suit the Southern routes they were destined to work.
I also understand that Stourbidge to Kidderminster was resignalled last year with a design specification to provide for a line speed increase to 85 mph.

 

[A-driver]

Not worked out why it is only 70mph. In a 377 you could easily get the speed down from 80 or 90 in time, and 455s would struggle to get much above 70-75 along there anyway.

I believe it's more to do with increased maintenance costs the higher speeds would bring due to the high amount if traffic combined with the increased power consumption which can cause issues in 3rd rail land.

Plus I doubt there would be much benefit to an increase as trains would start to catch others up and run on restrictive aspects more where the speed drops at new cross/bricklayers and Norwood junction.

The biggest mystery down that way to me was always why the down east grinstead between hurst green and Lingfield is cleared for 85 but the up is only cleared for 70. Both equally straight bits of track with equally Iong signal sections/sightings.

 

[JB25]

I believe it's more to do with increased maintenance costs the higher speeds would bring due to the high amount if traffic combined with the increased power consumption which can cause issues in 3rd rail land.

Plus I doubt there would be much benefit to an increase as trains would start to catch others up and run on restrictive aspects more where the speed drops at new cross/bricklayers and Norwood junction.

The biggest mystery down that way to me was always why the down east grinstead between hurst green and Lingfield is cleared for 85 but the up is only cleared for 70. Both equally straight bits of track with equally Iong signal sections/sightings.

I can't help you there but I was told the other day that the speed between Chipstead and Kingswood is different going up and down because the 20mph on the down line (up hill) was too slow for two cars in slippery conditions so they upped it to 30 on that line but it's still a painful 20 in the up direction.

 

[Ploughman]

I thought railway linespeeds were always the theoretical maximum safe speed rounded down to the nearest 5mph? That's what I was taught anyway.

That's what I was taught as well on a Track design Course at Derby about 8 years back.
You never round up only Down.

 

[edwin_m]

I believe it's more to do with increased maintenance costs the higher speeds would bring due to the high amount if traffic combined with the increased power consumption which can cause issues in 3rd rail land.

Plus I doubt there would be much benefit to an increase as trains would start to catch others up and run on restrictive aspects more where the speed drops at new cross/bricklayers and Norwood junction.

The biggest mystery down that way to me was always why the down east grinstead between hurst green and Lingfield is cleared for 85 but the up is only cleared for 70. Both equally straight bits of track with equally Iong signal sections/sightings.

It is probably also that the signals can be closer together for the lower speed and therefore trains can run closer together. I seem to remember from somewhere that 70mph gives about the maximum throughput of trains.

 

[Eagle]

Heading UP past Northam depot the line speed is shown as 80/85 (HST). (Where HST refers to XC Voyagers.)

Somewhat surprisingly, "HST" also includes 158s, 159s and Turbostars.

 

[Bald Rick]

I believe it's more to do with increased maintenance costs the higher speeds would bring due to the high amount if traffic combined with the increased power consumption which can cause issues in 3rd rail land.

Plus I doubt there would be much benefit to an increase as trains would start to catch others up and run on restrictive aspects more where the speed drops at new cross/bricklayers and Norwood junction.

The biggest mystery down that way to me was always why the down east grinstead between hurst green and Lingfield is cleared for 85 but the up is only cleared for 70. Both equally straight bits of track with equally Iong signal sections/sightings.

The main reason is signal spacing for the 'Southern Region' braking curve in the signal spacing standard. By chance I measured all the signals (off plan) a few weeks ag. If the Appendix 3 (or C) braking curve was used then another 10mph could be squeezed out in most places, but there are other things to consider and it saves less than a minute Croydon-London.

 

[ChiefPlanner]

A good number of speed restrictions depend on "previous history" - e.g vacuum braked train capabilty , and on one rural line I can think of - the risk of a passenger sticking his / her head out and nutting a bridge. Some of the reasons are lost in the mists of time - but local Route Investment Groups with the TOCS / FOCS and NR sitting together have lifted a good number of these by constructive challenge - particularly with track and switch replacement programmes and resignalling - the Sussex area of NR has been exceptionally pro active in sniffing out "easy" enhancements - though many other local area have been equally good at getting these niggling constraints lifted....(in many cases it reduces fuel / electricity consumption as well as smoothing out line speed profiles)

 

[The Planner]

Somewhat surprisingly, "HST" also includes 158s, 159s and Turbostars.

Which suggests that their braking characteristics match appendix C.

 

[455driver]

You also have Berriton tunnel on the Portsmouth direct line ,the old mk1 units used to fly through at 70 now everything seems to have to slow down to 40 i presume for clearance purposes

Buriton tunnel was 70 but because of the curvature of the tunnel and the increased length there was a chance of the 442s hitting each other if 2 passed in the tunnel so they were restricted to 40 through it. When the 444s and 450s were introduced it was decided to reduce the linespeed to 40 for all units because of the risk of the driver being in a 450 but having a 444 on the back or the driver forgetting what type of train they were driving as they both have the same cab.