Why 85mph?

[brad465]

Something I've noticed is with some lines, especially dead straight ones, either present or in history, the top line speed is 85mph.

I find this odd as a maximum value, where making it higher would a first glance not seem difficult. If the max was 90mph, while it not make a huge time difference (expect over long distances) it would seem more logical, or where dead straight (as in straight for much of its length), 100mph could be feasible, and would seemingly be more beneficial.

Usually though the signalling is 2 aspect where I've seen it, or single track, but a resignalling to 3 aspect (especially where there's old signalling due for replacement) would seemingly make this possible.

Examples where I know 85mph is the max speed include:

-Redhill-Tonbridge
-Liverpool-Manchester via Warrington Central (though electrification I believe saw a line speed increase to 90mph coincide)
-West of England Mainline (in places, especially on single track areas)

The Marshlink line also used to have 85mph, but was lowered to 60mph in conjunction with partial singling, but should I think have some work done if feasible to raise it again.

The Cotswold line also is a counter example, as 100mph is possible on single track parts.

 

[dggar]

Something I've noticed is with some lines, especially dead straight ones, either present or in history, the top line speed is 85mph.

I find this odd as a maximum value, where making it higher would a first glance not seem difficult. If the max was 90mph, while it not make a huge time difference (expect over long distances) it would seem more logical, or where dead straight (as in straight for much of its length), 100mph could be feasible, and would seemingly be more beneficial.

Usually though the signalling is 2 aspect where I've seen it, or single track, but a resignalling to 3 aspect (especially where there's old signalling due for replacement) would seemingly make this possible.

Examples where I know 85mph is the max speed include:

-Redhill-Tonbridge
-Liverpool-Manchester via Warrington Central (though electrification I believe saw a line speed increase to 90mph coincide)-West of England Mainline (in places, especially on single track areas)

The Marshlink line also used to have 85mph, but was lowered to 60mph in conjunction with partial singling, but should I think have some work done if feasible to raise it again.

The Cotswold line also is a counter example, as 100mph is possible on single track parts.

This is the CLC route. The Chat Moss route is the one that has been electrified.

 

[LNW-GW Joint]

Something I've noticed is with some lines, especially dead straight ones, either present or in history, the top line speed is 85mph.

There are probably several reasons, but one plausible theory is that 90mph brings in more stringent rules for things like level crossings.
To get 90mph you might need expensive new signalling, so 85mph with unaltered signalling is significantly cheaper.
Signal sighting also comes into it. You might need to move the signals for 90mph, which isn't worth the cost.
Stretches of the North Wales main line are like that.
Stretches of the Cambrian line are 85mph too, and that has many level crossings.

The CLC route is still 85mph (not sure why), it's the Chat Moss route which recently went to 90mph (except at the one remaining level crossing at Astley, which is 60mph).
Above 90mph the rules get stricter still, usually triggering a major upgrade.
There are some dead straight sections which are limited for other reasons, the lines over the fens for instance, because of unstable ground.
I'm sure an engineer will be along shortly to add some flesh to these bones!

 

[edwin_m]

Something I've noticed is with some lines, especially dead straight ones, either present or in history, the top line speed is 85mph.

I find this odd as a maximum value, where making it higher would a first glance not seem difficult. If the max was 90mph, while it not make a huge time difference (expect over long distances) it would seem more logical, or where dead straight (as in straight for much of its length), 100mph could be feasible, and would seemingly be more beneficial.

Usually though the signalling is 2 aspect where I've seen it, or single track, but a resignalling to 3 aspect (especially where there's old signalling due for replacement) would seemingly make this possible.

Examples where I know 85mph is the max speed include:

-Redhill-Tonbridge
-Liverpool-Manchester via Warrington Central (though electrification I believe saw a line speed increase to 90mph coincide)
-West of England Mainline (in places, especially on single track areas)

The Marshlink line also used to have 85mph, but was lowered to 60mph in conjunction with partial singling, but should I think have some work done if feasible to raise it again.

The Cotswold line also is a counter example, as 100mph is possible on single track parts.

85mph as a line speed does seem a bit odd, as there have been no trains (as far as I'm aware) with that as a top speed and there are no extra safety rules that kick in at just above that speed as there are with 100mph. Typically line speeds are pushed out to the maximum for the stock where possible on important lines where the operator thinks it would be worthwhile. Less important lines tend to be left at whatever speed they have always been.

However there are literally dozens of factors that need to be taken into account when looking to increase the speed. Some of them are:
- Curvature (probably the reason on the West of England)
- Signal Spacing or Signal Sighting (very expensive to change unless re-signalling is due)
- Ground conditions (an example being Stephenson's railway floating on Chat Moss - I wonder if Marshlink has the same problem?)
- Attainable speed (nearby station stops or speed restrictions make it not worthwhile to make a particular section faster)
- Limited clearances, so if the train went faster it might sway more and hit something.

No doubt someone else will be along with a few more!

I think the reason why the four very different lines mentioned should all have (or have had) the same speed is no more than coincidence.

 

[D1009]

Wasn't the "standard" line speed on the Southern Region 85 mph at one time?

 

[brad465]

However there are literally dozens of factors that need to be taken into account when looking to increase the speed. Some of them are:
- Curvature (probably the reason on the West of England)
- Signal Spacing or Signal Sighting (very expensive to change unless re-signalling is due)
- Ground conditions (an example being Stephenson's railway floating on Chat Moss - I wonder if Marshlink has the same problem?)
- Attainable speed (nearby station stops or speed restrictions make it not worthwhile to make a particular section faster)
- Limited clearances, so if the train went faster it might sway more and hit something.

Ground conditions could well be a factor on the Marshlink as crossing Romney Marsh, likely to be wet ground that needs frequent drainage, would lower stability. The land is also at or only just above sea level from Ham Street to Rye primarily.
--- old post above --- --- new post below ---

There are probably several reasons, but one plausible theory is that 90mph brings in more stringent rules for things like level crossings.
To get 90mph you might need expensive new signalling, so 85mph with unaltered signalling is significantly cheaper.
Signal sighting also comes into it. You might need to move the signals for 90mph, which isn't worth the cost.
Stretches of the North Wales main line are like that.
Stretches of the Cambrian line are 85mph too, and that has many level crossings.

I know level crossings definitely are a factor in 100+mph line speed limitations, the Bristol-Exeter line coming to mind especially, though 90mph might be less of an issue, though that might depend on the crossing types involved (especially ungated). A 5mph difference I would have thought meant signals either wouldn't have to be moved, or at least not very far, and for freight trains and non-multiple unit trains that need longer distances to stop could just have a lower speed for them enforced.

 

[MarkyT]

Crossings are an issue. 100MPH is the limit for AHBs but visibility at footpath and occupation crossings also play a part. Raising speeds might make the sighting time inadequate and force additional protection measures such as telephones or warning lights - costly on some rural routes with many such crossings.

There may also be a track maintenance issue too. Not my area of expertise, but for some rural routes it's simply not worth the extra maintenance costs of higher speed. If manufacturers could come up with new lighter train designs that could run at the higher speed with no more track wear then that could alter the equation.

 

[Bald Rick]

One day I'll do a sticky which explains line speed limitations, based on the standard.

There are almost 100 factors which have to be taken into account when setting a linespeed.

Track maintenance standards change at 90 for a given annual tonnage, so conceivably 85 keeps it below the higher standard.

 

[Starmill]

I thought the Chat Moss line had been upgraded from 75 to 90?

Is Slade Lane - Manchester Airport 75 or 85?

 

[richardio123]

i thought the chat moss line had been upgraded from 75 to 90?

Is slade lane - manchester airport 75 or 85?

75mph

 

[70014IronDuke]

Wasn't the "standard" line speed on the Southern Region 85 mph at one time?

exactly what I was thinking of posting. ISTR at the end of steam in 67, this was the official line speed for the LSWR main line. This when certain drivers were getting - on occasion - 105 MPH out of Merchant Navies in the last weeks of life.

 

[73001]

Wasn't the "standard" line speed on the Southern Region 85 mph at one time?

I think the class 33s were built with a max speed of 85mph which would fit with this as well. I always thought this seemed an odd speed to pick.

 

[Ploughman]

I think that track construction standards may also come into play.
As an example I believe jointed track is an automatic 90 limit.

 

[TDK]

I thought the Chat Moss line had been upgraded from 75 to 90?

The up Chat Moss is 90mph from before Earlsetown until the approaches to Astley where the speed drops to 75mph and then down to 60mph, after Astley it then increases to 75mph until Ordsall Lane where the speed across the junction is 25mph. It is the same in the down direction.

 

[Starmill]

I thought the 60 at Astley was temporary? Even if it is that's a disappointingly long stretch of remaining 75.

 

[craigybagel]

I thought the 60 at Astley was temporary? Even if it is that's a disappointingly long stretch of remaining 75.

It's a permanent board not a temporary one for the 60 at Astley so it looks like its here to stay. On the plus side, a large part of the current 75 section used to be 60, so although it's slower than the 90 sections of the line it's still an improvement over what it was.

Speaking of Chat Moss, I always thought it seemed strange that they went for 90 and not 100 when they upgraded the line, given almost all stock that works over it can do the higher speed. Apparently it wasn't worth the cost difference.

 

[FordFocus]

Over the routes I drive I can think of a few 85mph sections but the only reason I can come up with is because of the curvature of the corner, no signals or crossings nearby.

 

[Starmill]

It's a permanent board not a temporary one for the 60 at Astley so it looks like its here to stay. On the plus side, a large part of the current 75 section used to be 60, so although it's slower than the 90 sections of the line it's still an improvement over what it was.

Speaking of Chat Moss, I always thought it seemed strange that they went for 90 and not 100 when they upgraded the line, given almost all stock that works over it can do the higher speed. Apparently it wasn't worth the cost difference.

Not much seems to have been worth the cost difference! 'Oh we' do about half at 90 rather than 100 and leave the rest at 75, except this bit here that we will leave at 60 and hope nobody notices.' Hmm.

 

[Kentish Paul]

I think the class 33s were built with a max speed of 85mph which would fit with this as well. I always thought this seemed an odd speed to pick.

Yes, the class 33 maximum speed was 85mph. Always thought it odd. I will look up other "Southern" locos/units to see what their maximum speeds were.

 

[The Planner]

Astley LC was never part of the Chat Moss scheme, it was/has to be done seperately.

 

[Springs Branch]

Something I've noticed is with some lines, especially dead straight ones, either present or in history, the top line speed is 85mph.

I find this odd as a maximum value .......

It won't explain any historic manifestations, but for today, if you convert 140 km/h into "old money" and round to the nearest 5, it comes out as 85 mph.

I don't know if this has any significance whatsoever, just an observation.

 

[Loop & Link]

Not much seems to have been worth the cost difference! 'Oh we' do about half at 90 rather than 100 and leave the rest at 75, except this bit here that we will leave at 60 and hope nobody notices.' Hmm.

There's not much of it that's 75 between Newton-le-Willows just the approaches to Astley so a short distance. I expect the full 90 isn't in place between Astley and Eccles is to do with Barton Moss.

 

[Senex]

There's not much of it that's 75 between Newton-le-Willows just the approaches to Astley so a short distance. I expect the full 90 isn't in place between Astley and Eccles is to do with Barton Moss.

Does anyone know how we deal with Chat-Moss-type land in this country? A great fuss was made about getting it up to 75 from 60, but that doesn't seem too impressive alongside Stilton Fen's 105 on the GN main line. But what really makes me wonder is what has been done in Germany on several of the main lines radiating out from Berlin which have very similar ground problems that led to very severe PSRs in the past. But in most cases formation works seems to have allowed these to be brought up to line speed. Do they do things differently?

 

[AM9]

Does anyone know how we deal with Chat-Moss-type land in this country? A great fuss was made about getting it up to 75 from 60, but that doesn't seem too impressive alongside Stilton Fen's 105 on the GN main line. But what really makes me wonder is what has been done in Germany on several of the main lines radiating out from Berlin which have very similar ground problems that led to very severe PSRs in the past. But in most cases formation works seems to have allowed these to be brought up to line speed. Do they do things differently?

Maybe similar but not necessarily the same. You've mentioned Chat Moss and Stilton Fen in the same posting. I suspect that they are both different enough to account for their respective limits. Then there is the effect on the route either side, i.e. the ECML is predominally a 125mph line. A 60 or 75mph chunk in the middle of that has a much bigger impact of services than 75mph on a 90mph line which is less than 25 miles of open track between much more restrictive speed limits.

 

[edwin_m]

HS1 over Dagenham Marshes is carried on concrete, effectively a ground-level viaduct. If the ground is soft a train creates a "bow wave" that limits its safe speed, so the faster the line the stiffer the formation needs to be.

 

[30907]

exactly what I was thinking of posting. ISTR at the end of steam in 67, this was the official line speed for the LSWR main line. This when certain drivers were getting - on occasion - 105 MPH out of Merchant Navies in the last weeks of life.

It was the maximum permitted on any SR route for steam trains (or do I mean locohauled? Haven't got an old WTT handy on holiday!), with 75 for electrics until the Bournemouth electrification when it was raised to 90 generally.

 

[southern442]

Oxted line is also 85 mph in quite a few places too.

 

[Bevan Price]

I think that, in the steam era, most speed limits had a big safety margin because, until the 1950s, the majority of steam locos did not have speedometers.

 

[Class 170101]

I think that, in the steam era, most speed limits had a big safety margin because, until the 1950s, the majority of steam locos did not have speedometers.

Surely that could be argued until the days of OTMR and TPWS as speed limits would have been difficult to enforce?

 

[brad465]

Oxted line is also 85 mph in quite a few places too.

When I last went down that line I don't remember seeing anything better than 70, though I'll take your word for it