Question about line speeds.

[Class325Guy]

So I am not from the UK, but I have an interest in trains from the UK and had a question about track speeds. So obviously things like the sharpness of curves, crossings and switches will affect track speeds. But when the difference in track speeds is small, what kind of things are taken into account when determining the speed? For example, on some routes the speed will go from 90 to 95, or 100 to 110. Besides how sharp the line curves, what other things are taken into consideration when say allowing a train to travel at 90 vs 95 or 100 vs 110? I fell like and extra 5mph wouldn't make that much of a difference, but what do I know.

Thanks

 

[Annetts key]

Track formation, strength of structures like bridges, signal spacing (distance between them) and hence trains are able to have enough braking distance (service braking distance). In some cases, a slight reduction in line speed may be considered instead of adding extra signals if the minimum required sighing distance cannot be achieved.

With curves, the cant (angle of tilt) of the track that can be applied. This is always a compromise if different trains may be travelling at different speeds, maybe due to some stopping at a station.

There will be other things I can’t think of right now…

 

[Snow1964]

Some restrictions are due to structures, old bridges etc where the dynamic load or clearance requires a limit

There is also history and what trains operate it, there isn't lot of sense in upgrading a line to a higher speed if nothing will use it. Many main lines in the South were 90mph because that was the maximum of most electric trains until 20 years ago. The distances between stations has meant little need to have 100mph trains

In busy areas, often governed by how close signals are, as lower speed limit requires lower braking distance

 

[O L Leigh]

Line speed can also be affected by signal spacing, especially where signals are not uniformly spaced. One of the routes I sign has this, where the distant signal (which will give you the yellow to warn of a red at the next signal) is closer to the associated stop signal. This means that the line speed has to be adjusted on approach to the distant to ensure compliance with braking rates so that all trains of whatever type can stop at the red.

 

[Bald Rick]

There are over 80 reasons why a linespeed may be set (or put another way, restricted). Horizontal Alignment (curvature) is but one.

The overall ‘headings’ under which these 80 fall are:

Track, Signalling, Overhead Line (or conductor rail, or both), Power supply, Level Crossings, Structures, Gauging, Aerodynamics, Stations, Maintenance considerations, Operational considerations. And probably a couple more I can’t remember!

 

[30907]

Historically, each route had an overall linespeed which for double-track main lines was the equivalent of the road "national speed limit" (but set by the Chief Civil Engineer of the railway/region - ISTR that for a few years the Western Region had no upper limit); lower restrictions were applied as necessary (for any of Bald Rick's 80 reasons).
The nominal linespeed would rarely have been reached on some routes - for example, Eastleigh/Redbridge to Salisbury was the SR standard 85mph for hauled trains (75 for MUs), but I doubt they exceeded 60 very often.
I wonder if some upper limits have simply never been updated?

I mention this particular example because the "line limit" on that route is still 85:
https://www.railforums.co.uk/threads/trivia-most-useless-speed-limits.239459/#post-5931334

I have occasionally wondered whether some 95mph limits were introduced in response to the arrival of the Brush Type 4 aka cl47 whose top speed was just that.

Happy to be proved wrong!

 

[hexagon789]

I have occasionally wondered whether some 95mph limits were introduced in response to the arrival of the Brush Type 4 aka cl47 whose top speed was just that.

Ipswich to Norwich was raised from 90 to 95 initially, when Shenfield to Ipswich was raised to 100mph in 1972. Precisely because of the top speed of 47 being 95mph and those being used more-or-less exclusively Ipswich to Norwich, so linespeed was not raised any further at that point. On the Clacton services, the 309s wwre 100mph units, hence linespeed south of Isowich being raised from 90 to 100 to permit use of their top speed officially.

It was increased to the full 100 north of Ipswich when electrification was extended to Norwich.

Carstairs-Edinburgh is 95mph for the same reason; 47s were the main traction. Previously linespeed was 70mph.

 

[Bald Rick]

I have occasionally wondered whether some 95mph limits were introduced in response to the arrival of the Brush Type 4 aka cl47 whose top speed was just that.

Typically, 95mph is chosen as a limit as certain track standards go up a notch at 100mph. This is in terms of maintenance / inspection frequencies, but in some cases (in the old days) specification of track components.

And of the trains that use it can’t get to 100mph anyway (or only just), then there’s no point! The benefit of 100mph over 95 is 1.9 seconds per mile, so you need 32 uninterrupted miles at 100 to save a minute compared to 95.

 

[Sorcerer]

In the case of the West Coast Main Line specifically line speeds can also be determined by the permissible tilt on different corners. As I understand it the tight corner at Leighton Buzzard has two EPS limits, that being 125 for Pendolinos, and 115 for Super Voyagers due to the slightly lower degree of tilt with the latter. Regular trains meanwhile only run at 90.

 

[Annetts key]

One of the things I forgot to mention in my earlier post - tunnels. With some there is a lower linespeed through the length of the tunnel compared to the plain line track either side.

 

[30907]

Typically, 95mph is chosen as a limit as certain track standards go up a notch at 100mph. This is in terms of maintenance / inspection frequencies, but in some cases (in the old days) specification of track components.

Glad to have been corrected.