What determines the maximum safe speed?

[DynamicSpirit]

A question that randomly popped into my head... What, in engineering terms, is it that limits the maximum speed that trains can travel on any particular stretch of line? If you wanted to upgrade a section of line to allow higher speeds, what would you typically need to change?

Clearly a big factor is going to be the radius of any curves: Straighter = faster because you want to limit the sideways force as trains go round the curves; but I assume there must be other things because I can think of places where track seems to be reasonably straight but where trains usually go fairly slowly, so I'd presume something else is limiting the maximum speed.

I'm asking in general - not thinking of any particular locations.

 

[colchesterken]

I have often thought about that, the speed limit is 40 from Liv St to well passed Bethnal Green, When it seems to me 60 would be possible

 

[Krokodil]

Signal spacing. If linespeeds are higher, you need a longer distance between the first caution aspect (yellow in three aspect areas, double yellow in four) and the stop aspect.

 

[Bevan Price]

Condition of track, or trackside infrastructure.
Design of trains. Can anyone imagine ride quality of Pacers if they had been capable of reaching 100 mph ???

 

[Diedinium]

A question that randomly popped into my head... What, in engineering terms, is it that limits the maximum speed that trains can travel on any particular stretch of line? If you wanted to upgrade a section of line to allow higher speeds, what would you typically need to change?

Clearly a big factor is going to be the radius of any curves: Straighter = faster because you want to limit the sideways force as trains go round the curves; but I assume there must be other things because I can think of places where track seems to be reasonably straight but where trains usually go fairly slowly, so I'd presume something else is limiting the maximum speed.

I'm asking in general - not thinking of any particular locations.

There's the track quality and type as well: you can't run faster services over worn track (too bumpy, larger risk of derailment) and ideally you need to replace jointed rail with continuously welded rail as well. The actual quality of the track bed makes a difference as well, HS2 for example is using "slab track" (part of why the costs are so high...) as it's better able to maintain the position of the rails over longer periods of time and higher volumes of traffic. It's no good having the best put together track if every few weeks it would need correcting due to shifting of the rails.

Not to mention points, I don't know the specifics, but for faster speeds you generally need different designs - especially for the frogs (the part of the point that moves)

Like you were saying, part is to do with the curvature and cant of the track as well: you can have a really heavily curved track if the cant is higher to alleviate the sideways forces and still run faster services over it, but only to a certain extent as it still needs to be safe for the train to go over at lower speeds without requiring the downwards force to keep it on the track. This is why tilting trains are a thing, as why tilt the track if you can get the train to create it's own tilt as needed - it's the uncomfortable sideways forces that prevent a lot of higher speeds after all, which the tilting helps to eliminate.

 

[edwin_m]

Even when you have a tilting train, the cant serves a purpose because it reduces the wear on the outer rail and the wheel flanges. Conversely if too many trains pass by at lower than the equilibrium speed, there will be more wear on the inner rail.

High speed lines use "swing nose crossings", which move with the switches to close the gap you get in a normal crossing.

I think someone posted on here that there are about 80 factors to be considered when looking to increase speeds. A few not mentioned yet:

• Ground conditions
• Condition of under-track structures
• Overhead wire gradient
• Dynamic clearances
• Signal sighting
• Safe distances for trackside staff
• Track separation (risk of ballast being thrown up when two trains pass)
• Cross-section area of tunnels

 

[hexagon789]

As I understand, going off memory of a previous thread on here, that there are over 80 factors that can govern the posted speed limit at any one point on the network.

Among those I can recall:

Curvature
Transitions
Signal spacing
Signal sighting
Signal overlaps
Headway
Track condition
Track construction
Jointed Track
Complex S&C
Junction layouts/signalling
Level crossings
Train capability
Formation
Clearances
Structures
Earthworks
Tunnels (ie - aerodynamic pressure)
Cant
Negative cant (Carstairs pre-remodel for example)
Vertical curvature

And many, many more

 

[Bald Rick]

A question that randomly popped into my head... What, in engineering terms, is it that limits the maximum speed that trains can travel on any particular stretch of line?

There are over 80 things to consider. I wont list them!

As I understand, going off memory of a previous thread on here, that there are over 80 factors that can govern the posted speed limit at any one point on the network.

Is the right answer.

I have often thought about that, the speed limit is 40 from Liv St to well passed Bethnal Green, When it seems to me 60 would be possible

Signal spacing and sighting, Fixed tension OLE, and avoiding approach control on the signalling are the main reasons.

 

[Val3ntine]

Upper Holloway - South Tottenham is a very good example (30mph limit). I assume likely due to track condition perhaps.

 

[Annetts key]

track condition.

In general, the type and quality of the track, including the type of rail, type of sleepers, type, condition and depth of ballast, and the formation that the track sits on, all are factors.

Signalling has been mentioned. The type of signalling is a big factor. Because line capacity also is part of the equation where conventional signalling is used. It’s a four way compromise between the maximum speed and hence the service braking distance of what types of train you intend on running, the number of trains per hour and cost and complexity.

And it’s expensive to change any of the above on an existing line…

 

[baz962]

Upper Holloway - South Tottenham is a very good example (30mph limit). I assume likely due to track condition perhaps.

Horrible around there. They had a very long 20 mph tsr on there one winter

 

[dk1]

I have often thought about that, the speed limit is 40 from Liv St to well passed Bethnal Green, When it seems to me 60 would be possible

As said that’s down to signal spacing/sighting. The DM is 35mph at Bethnal Green for this same reason.

 

[Deepgreen]

Condition of track, or trackside infrastructure.
Design of trains. Can anyone imagine ride quality of Pacers if they had been capable of reaching 100 mph ???

That's a rolling stock limitation, rather than track/line.

This question popped into my head at the start of LORO services to Clapham Junction, where approaching or leaving the station they are subject to a 10mph limit for what seems like about 0.75 miles - a frustrating crawl when SW trains are speeding by.

In general, though, the permanent line speed is essentially determined by signal spacing (including the number of aspects) and route formation (curvature). Obviously other speed limits arise through track condition, civil engineering issues, etc.

 

[Purple Train]

Upper Holloway - South Tottenham is a very good example (30mph limit). I assume likely due to track condition perhaps.

Isn't it longitudinal timbers around there?

 

[colchesterken]

I would have thought that modern brakes ie re gen, or reostatic and discs would allow much higher speeds within existing signals
I was on a refurb 321 near Witham at full speed when it did an emergency stop
I assume it was a false alarm as we were off after a few mins
It stopped far quicker than I thought possible for a train at speed

 

[Annetts key]

I would have thought that modern brakes ie re gen, or reostatic and discs would allow much higher speeds within existing signals
I was on a refurb 321 near Witham at full speed when it did an emergency stop
I assume it was a false alarm as we were off after a few mins
It stopped far quicker than I thought possible for a train at speed

Yes, that’s why there are differential speed limits on some lines / sections.

However, if it’s a mixed traffic line (that is freight or conventional passenger traffic uses it or may use it, including steam trains), then the infrastructure including the signalling has to be based on the trains having conventional braking systems.

Where possible, a higher differential speed limit for trains with modern braking systems can then be used. But there are practical limits. Because we are back to the four way compromise I talked about earlier in post #10.

 

[Bald Rick]

I would have thought that modern brakes ie re gen, or reostatic and discs would allow much higher speeds within existing signals
I was on a refurb 321 near Witham at full speed when it did an emergency stop
I assume it was a false alarm as we were off after a few mins
It stopped far quicker than I thought possible for a train at speed

Thats all about signal spacing. 9%g service brake (12%g emergency), and the speeds it enables has been a thing since 1976.

 

[edwin_m]

Thats all about signal spacing. 9%g service brake (12%g emergency), and the speeds it enables has been a thing since 1976.

But there are still older trains around that can't achieve this and the signalling has to take account of the worst case unless those trains are banned or subject to specific restrictions. Aren't 150s and 156s (tread braked) only 7%g? I believe 12% emergency only came in around 1994 (part of the SPAD Reduction and Mitigation programme I was involved in), and only for units where it was feasible to do it without major modifications.

With ETCS each train knows its own braking performance and the state of the line ahead, so can travel at the most appropriate speed when approaching a limit of movement authority (a red signal in old parlance) rather than being constrained by fixed signal positions and aspects. But all the other factors determining maximum permitted speed will apply.

 

[Bald Rick]

But there are still older trains around that can't achieve this and the signalling has to take account of the worst case unless those trains are banned or subject to specific restrictions.

indeed.

I believe 12% emergency only came in around 1994

I thought the PEPs could do it, and I’ve defintiely been on an HST that braked at at least 12%g, stuff was flying off the tables!! It might not have been formalised in 1976 of course, my point was that differential speed limits that take into account better braking perfromance have been around since then.

 

[bahnause]

With ETCS each train knows its own braking performance and the state of the line ahead, so can travel at the most appropriate speed when approaching a limit of movement authority (a red signal in old parlance) rather than being constrained by fixed signal positions and aspects. But all the other factors determining maximum permitted speed will apply.

ETCS has the disadvantage of extremely restrictive braking curves. Even with conventional signalling, a lot of countries use graduated speeds according to the braking capacity of the trains. This must of course already be taken into account in timetable planning.

 

[edwin_m]

I thought the PEPs could do it, and I’ve defintiely been on an HST that braked at at least 12%g, stuff was flying off the tables!! It might not have been formalised in 1976 of course, my pont was that differential speed limits that take 8nto account better braking perfromance have been around since then.

From what I recall of discussions circa 1994 (I was the humble minute taker in some meetings involving very senior people!) various classes were capable of 12%g braking, but at the time the emergency brake was set up to give the same deceleration as the service brake but in a "failsafe" way so it would still work in the event of defects that inhibited service braking.

 

[greyman42]

What is the maximum speed of the Selby to Hull line? It is very straight but i don't believe trains can travel at their maximum speed regardless of stopping patterns. Is the problem subsistence?

 

[hexagon789]

What is the maximum speed of the Selby to Hull line? It is very straight but i don't believe trains can travel at their maximum speed regardless of stopping patterns. Is the problem subsistence?

90mph both normal and some sections with lower normal limits but higher SP limits, again up to a 90mph ceiling.

There are a lot of speed restrictions, even by sometimes only 5mph at times (e.g. from 90 to 85/SP90 or 70/SP75 to 70 etc), along the line.

 

[dk1]

I was once told that there’s a lot more cost/inspection involved for going that extra 10mph to 100 and why so many regional routes are capped at 90.

 

[Bald Rick]

I was once told that there’s a lot more cost/inspection involved for going that extra 10mph to 100 and why so many regional routes are capped at 90.

Yes, 90mph is the boundary between Cat 3 and Cat 2 maintenance standards on low-medium traffic routes (basically anything with less than about 24 coaches an hour or a freight every two hours, in a typical 18 hour service day). Cat 2 (the higher standard) requires twice the level of visual inspection and ultrasonic rail testing of Cat 3.

 

[dk1]

Yes, 90mph is the boundary between Cat 3 and Cat 2 maintenance standards on low-medium traffic routes (basically anything with less than about 24 coaches an hour or a freight every two hours). Cat 2 (the higher standard) requires twice the level of visual inspection and ultrasonic rail testing of Cat 3.

Many thanks. Was told that years ago but often wonder whether things come to me through dreams lol.