ScotTrainLad
Member
Just interested in how speed limits were set.
-
Our booking engine at tickets.railforums.co.uk (powered by TrainSplit) helps support the running of the forum with every ticket purchase! Find out more and ask any questions/give us feedback in this thread!
How are speed limits set?
- Thread starter ScotTrainLad
- Start date
Sponsor Post - registered members do not see these adverts; click here to register, or click here to log in
R
RailUK Forums
Bevan Price
Established Member
- Joined
- 22 Apr 2010
- Messages
- 7,343
Primarily safety. For example.
Go too fast round sharp curves or pointwork, and train might "fall off".
Signal sighting distances.
Potentially hazardous level crossings.
And maybe sometimes, to reduce track wear. (Lower speeds usually reduce wear.)
CEN60
Member
- Joined
- 17 Dec 2018
- Messages
- 267
There are a suite of standards that dictate track speed based on curvature, speed, cant, other associated geometry and material types. However just because the geometry etc can support speed does not necessarily mean the trains travel at that speed. There are many other factors that may constrain speed - for instance - signalling and issues with weak structures to name but two.
Last edited:
bunnahabhain
Established Member
- Joined
- 8 Jun 2005
- Messages
- 2,070
Some limits between Chinley and Hazel Grove have just been reduced based on what is actually achievable versus what you *could* do. I seem to recall Railtrack could charge more for a 90mph stretch of line than a 70mph stretch so you ended up with some random 90mph bits on otherwise 70mph railways. Chinley to Bugsworth being a good example.
Revilo
Member
- Joined
- 13 Jan 2018
- Messages
- 280
If we’re talking speed limits on roads, parish councils have an input so they can be quite subjective and based on local politics.
CarrotPie
Member
Considering this is in the Railway Discussion section, I doubt we are...
On the OP's point, TOCs also have an input if they need/want a PSR (permanent speed restriction) to be increased for their timetables.
ptreanor
Member
- Joined
- 2 May 2017
- Messages
- 38
Braking distances. The right to speed is dictated by the ability to stop!
Railsigns
Established Member
- Joined
- 15 Feb 2010
- Messages
- 2,503
Tail wagging the dog. Permissible speed usually determines braking distances, not the other way around.
ptreanor
Member
- Joined
- 2 May 2017
- Messages
- 38
That's not how signalling works...
Railsigns
Established Member
- Joined
- 15 Feb 2010
- Messages
- 2,503
ptreanor
Member
- Joined
- 2 May 2017
- Messages
- 38
OK, let's be pedantic. To run a single captive train at any speed does not require a signalling system but that wasn't my point. I'm a signalling engineer by the way...
Former railway companies used to state speed limits in the working timetables, but the GWR was the only one to state on certain main lines "the speed may be as high as required".
It normally was wholly up to the driver's judgement anyway, as most steam locomotives (and all before about the 1930s) were not fitted with speedometers, so you really did not have any idea of the actual values attained.
It normally was wholly up to the driver's judgement anyway, as most steam locomotives (and all before about the 1930s) were not fitted with speedometers, so you really did not have any idea of the actual values attained.
Which was exactly the mistake that the German Maglev trial system with just one train made, culminating in their disastrous accident.
td97
Established Member
- Joined
- 26 Jul 2017
- Messages
- 1,299
Yes, based on driveability and also consideration for standardising line speeds in both directions to reduce driver workload.
Deafdoggie
Established Member
- Joined
- 29 Sep 2016
- Messages
- 3,092
What comes first? Do we say "we want a 100mph railway, design the signals to cope with it" Or do we say "This is where the signals are, how fast can we go?"
CarrotPie
Member
We say the former (and we want this timetable with these trains so make sure it can accommodate that too please).
renegademaster
Member
How long the blocks are , are based on the speed/stopping distance are they not?
QueensCurve
Established Member
- Joined
- 22 Dec 2014
- Messages
- 1,914
Speed limits around bends are set for passenger comfort. Hence when BR resarch showed that it was possible for trains to go 40% faster round the bends without risk of derailment, they proposed the Advanced Passenger Train with tilt so that it could go 40% faster round the bends.
They also increased speed limits for regular trains at a point in the 1980s I recall, so the Colwich-Cheadle Hulme line speeds increased from 85mph to greater numbers such as 95mph.
Then someone looked at it after a number of years, and imposed a lower speed restriction on the down line between Adlington and Poynton because the signal sighting was poor. In due course this restriction was lifted again, but only because the signal sighting issue had been resolved in some way.
So it's a complex interplay between various factors, clearly.
Railsigns
Established Member
- Joined
- 15 Feb 2010
- Messages
- 2,503
Almost certainly not. If such a thing did exist, how would it account for cant?
Depending on the location, signal Sighting and curvature of lines I assume would dictate the threshold of a maximum speed. Example, 50mph left hand curve - 55mph or 60mph would probably not have an effect on say Passanger comfort. Would there be a set threshold in this example but reduced by what...
another problem is if there room for a well laid out transition curve (where the radius get gradually smaller or greater each side of a curve. Too short a transition curve would cause the train to lurch so a slower speed would be better. Stations, junctions and structures could affect transition curves.
You could have a tool which helped you to measure the horizontal radius of a curve. Many years ago I made a crude tool with greaseproof paper (semi-transparent), a pencil and a pair of compasses, when I was interested in road design. You could then see what was the cant required for any specified speed or vice versa, and the maximum speed assuming maximum cant.
Bevan Price
Established Member
- Joined
- 22 Apr 2010
- Messages
- 7,343
I think that was the case on some lines from the 1960s. First generation units were restricted to 70 mph, so some line limits were reduced tp 70 mph (e.g. Wigan / Southport.)
CEN60
Member
- Joined
- 17 Dec 2018
- Messages
- 267
Please also remember that rolling stock plays a part too - on the West Coast in Scotland there are parts of the Route that have 4 different speed restrictions depending on the vehicle that is using it - Pendolinos, Tilting Voyagers (I appreciate they no longer tilt), conventional non tilt passenger & Freight. Also note that the NR Standard changed not that long ago to allow conventional non tilt passenger stock in some locations (depending on the type of rail & sleepers etc) to run faster than was previously allowed. In other words “it’s complicated” and lot and lots of different infrastructure from different disciplines may constrain the theoretical max speed the track alone might support!
hexagon789
Veteran Member
Avanti 221s still tilt.
CEN60
Member
- Joined
- 17 Dec 2018
- Messages
- 267
Good to know - still a pain in the sectional appendix and when you do the speed calcs!