Speed Limits

Status
Not open for further replies.

cjp

Member
Joined
28 Jan 2012
Messages
1,060
Location
In front of a computer
The Royal Albert Bridge at Saltash started this train of thought. but I now have a couple of questions for the gurus.

The Royal Albert Bridge has a 15mph speed limited and what sounds like jointed track. What science determines speed limits?
Or are speed limits just a finger in the wind and being cautious (like one sees on roads that have just been sprayed and gritted @ 5, 10 or 15mph)?:idea:

What would be the harm in running trains on the bridge at say 35mph especially if the vibration or percussive effects of jointed track was eliminated by laying welded track?

And do speed limits depend on the weight of the trains ie light trains (electric) can go faster than heavy freight pulled by diesel laden locos. (I know braking distance and signal intervals is also a factor).

Finally I thought the limit on speeds was the radius of curves and the cant of the track rather than anything else but i guess it is combination of .. . .

I am interested in any answers but especially about the low speed limit Brunel's bridge's.
http://www.squidoo.com/Royal-Albert-Bridge
 
Sponsor Post - registered members do not see these adverts; click here to register, or click here to log in
R

RailUK Forums

Joined
6 Sep 2011
Messages
931
Location
Blackpool south Shore
I believe it used to be 15mph on the approaches (because of the curves) and 20mph on the bridge. It has been raised in the 70's 80's when they extended the single line to the other side of the SR, to save replacing both iron bridges over the now Gunnislake track.
 

Bald Rick

Veteran Member
Joined
28 Sep 2010
Messages
14,033
There are nearly 100 separate reasons for speed restrictions, less than half of which relate to what is beneath the train wheels. In this case I guess it is to do with the structural capacity of the bridge itself combined with the curves.
 

Joseph_Locke

Established Member
Joined
14 Apr 2012
Messages
1,868
Location
Within earshot of trains passing the one and half
There are nearly 100 separate reasons for speed restrictions, less than half of which relate to what is beneath the train wheels. In this case I guess it is to do with the structural capacity of the bridge itself combined with the curves.
Now there's a challenge:
Permanent:
Above the track
Junction complexity (Manchester Piccadilly)
Signal sighting
Braking distance
Class of signal route
Structure clearances
OLE wire gradient
OLE construction
Conductor rail ramp types
Electrification changeovers
Tunnel aerodynamics
Level crossing restrictions
SSI boundary issues

Track:
Cant deficiency
Rate of change of cant or cant deficiency
Vertical curve radius
Ballast / formation condition
Rail wear / rail depth
Rail stress condition
Rail age / rail defects
Rail section
Sleeper type
Sleeper spacing
Fastening type
Renewals works affecting stability
Temporary alignments
Switch type (full vs. shallow, e.g.)
Check rail entry flare lengths in S&C

Below the track:
Bridge RA rating
Bridge resonance
Formation critical velocity (Chat Moss)
High risk earthworks
Mining subsidence
Sub-surface construction

Other
Limited by specific traction or rolling stock capability
Limited by class-specific braking performance


Temporary:
Single line working
Adjacent worksites
High winds
High rail temperatures
Conductor icing
Fog
Trespassers / line inspection
Authorised to pass at Danger by Signaller
Measuring and recording (e.g. UTU)
Out of Gauge loads / special arrangements

How am I doing?
 

Zoe

Established Member
Joined
22 Aug 2008
Messages
5,905
Wouldn't rail defects be subject to a temporary speed restriction until the track is repaired rather than a new permanent speed restriction?
 

TDK

Established Member
Joined
19 Apr 2008
Messages
4,126
Location
Wrexham
Now there's a challenge:
Permanent:
Above the track
Junction complexity (Manchester Piccadilly)
Signal sighting
Braking distance
Class of signal route
Structure clearances
OLE wire gradient
OLE construction
Conductor rail ramp types
Electrification changeovers
Tunnel aerodynamics
Level crossing restrictions
SSI boundary issues

Track:
Cant deficiency
Rate of change of cant or cant deficiency
Vertical curve radius
Ballast / formation condition
Rail wear / rail depth
Rail stress condition
Rail age / rail defects
Rail section
Sleeper type
Sleeper spacing
Fastening type
Renewals works affecting stability
Temporary alignments
Switch type (full vs. shallow, e.g.)
Check rail entry flare lengths in S&C

Below the track:
Bridge RA rating
Bridge resonance
Formation critical velocity (Chat Moss)
High risk earthworks
Mining subsidence
Sub-surface construction

Other
Limited by specific traction or rolling stock capability
Limited by class-specific braking performance


Temporary:
Single line working
Adjacent worksites
High winds
High rail temperatures
Conductor icing
Fog
Trespassers / line inspection
Authorised to pass at Danger by Signaller
Measuring and recording (e.g. UTU)
Out of Gauge loads / special arrangements

How am I doing?
I have never heard of Fog being an issue with speed.
You also have flooding, snow, train defect speed restrictions.
Passing a signal at danger does not have a value, it is drive according to instruction to stop short of an obstruction.
You have also got Temporary Block working
These are temporary or Emergency speed restictions Ballast / formation condition
Rail wear / rail depth
Rail stress condition
Rail age / rail defects

The ones below I have not heard of
Conductor rail ramp types - I belive that these are installed taking the current speed into account

Most permanent speed restriction come under signal sighting (spacing) amount of signal aspects, curvature of the track, clearance and guage, train type (differential) the speed restiction for various train types have a letter code generally and this can be SP (sprinter), HST (high speed train that also incorporates many other stock), EPS (enhanced for tilting), MU (multiple unit) there are others regionally as well. Apart from that an excellent description and many of those I have never heard of as I am not in the rail engineering group.
 

Joseph_Locke

Established Member
Joined
14 Apr 2012
Messages
1,868
Location
Within earshot of trains passing the one and half
I have never heard of Fog being an issue with speed.
Perhaps not Fog recently, but there was a period recently where the WCML had a talked-by at 20mph due to drifting smoke.

TDK said:
The ones below I have not heard of
Conductor rail ramp types - I belive that these are installed taking the current speed into account
But they can limit speed - bad example perhaps.

I missed minimum reading time for signals, thanks for that one.
 

notadriver

Established Member
Joined
1 Oct 2010
Messages
3,363
Interesting but it's true that trains could be cautioned past an area with heavy drifting smoke ie from a bonfire but fog specifically - definitely no restrictions.
 

HSTEd

Veteran Member
Joined
14 Jul 2011
Messages
11,726
Would this mean that a continuous cab signalling system, even simply overlaid over the existing signalling system would enable significant increases in some speed limits just as a result of eliminating the sighting issues?
 

Bald Rick

Veteran Member
Joined
28 Sep 2010
Messages
14,033
Cab signalling removes the need for any permanent speed restrctions required for pure signalling purposes, with one or two exceptions. But there's still all the others to think about before the speed can be raised.
 

John55

Member
Joined
24 Jun 2011
Messages
800
Location
South East
On plain track isn't the most important speed restriction passenger comfort as demonstrated by the use of Pendolinos on the WCML to increase running speed.
 

cjp

Member
Joined
28 Jan 2012
Messages
1,060
Location
In front of a computer
There are nearly 100 separate reasons for speed restrictions, less than half of which relate to what is beneath the train wheels. In this case I guess it is to do with the structural capacity of the bridge itself combined with the curves.
I have seen the list and it is impressive but it does not answer my question about how the limits are set just the why limits are set.

Let me assume that resonance is an issue with the bridge I first wrote about, Does someone do some pretty heavy calculations and made a number of assumptions (as they perhaps lack technical drawing or "as built drawing" and come up with a figure ) Or does someone say"Resonance could be an issue lets slow down the trains so they can quickly stop if something goes wrong"?
I could see the second approach (being what I would call "finger in the wind") being the case if there was mining subsidence for example and who knows what might be happening underground.
 

Yew

Established Member
Joined
12 Mar 2011
Messages
4,237
Location
Nottingham
On plain track isn't the most important speed restriction passenger comfort as demonstrated by the use of Pendolinos on the WCML to increase running speed.
The increase in ride quality is more due to the billions spent on track improvements than pendolinos
 

Bald Rick

Veteran Member
Joined
28 Sep 2010
Messages
14,033
I have seen the list and it is impressive but it does not answer my question about how the limits are set just the why limits are set.

Let me assume that resonance is an issue with the bridge I first wrote about, Does someone do some pretty heavy calculations and made a number of assumptions (as they perhaps lack technical drawing or "as built drawing" and come up with a figure ) Or does someone say"Resonance could be an issue lets slow down the trains so they can quickly stop if something goes wrong"?
I could see the second approach (being what I would call "finger in the wind") being the case if there was mining subsidence for example and who knows what might be happening underground.
In the case of the Royal Albert Bridge, indeed all structures, the speed (and axle load) limit insofar as structures are concerned will be set by applying first principles calaulations to the drawings (which are available for the R.A.B). In the event that original drawings are not available, the calcs are stilldone but based on what exists on the ground.


Bridge resonance isn't an issue at low speed, nor on bridges without ballast. It is a curious subject which only reared it's head in this country on the West Coast upgrade. In simple terms, on certain types of metal decked bridges, at certain linespeeds with certain trains, the ballast has a tendency to act like a liquid, and over time migrate off the bridge deck.
 

KA4C

Member
Joined
7 Mar 2012
Messages
403
I believe it used to be 15mph on the approaches (because of the curves) and 20mph on the bridge. It has been raised in the 70's 80's when they extended the single line to the other side of the SR, to save replacing both iron bridges over the now Gunnislake track.
It is 15 mph on the bridge
 

TDK

Established Member
Joined
19 Apr 2008
Messages
4,126
Location
Wrexham
I have seen the list and it is impressive but it does not answer my question about how the limits are set just the why limits are set.

Let me assume that resonance is an issue with the bridge I first wrote about, Does someone do some pretty heavy calculations and made a number of assumptions (as they perhaps lack technical drawing or "as built drawing" and come up with a figure ) Or does someone say"Resonance could be an issue lets slow down the trains so they can quickly stop if something goes wrong"?
I could see the second approach (being what I would call "finger in the wind") being the case if there was mining subsidence for example and who knows what might be happening underground.
Signal sighting (distant signals) - the speed is set with the sevice braking distace of the lowest brake force traction taken into account

Signal spacing - same as above

Curvature of track - speed set below the advised speed that a train could derail

Clearance of structures - set so a train will not collide with the structure

Temporary Speed restrictions - set on the basisi of the condition of the track can be anything from 5 mph to 120mph - these are not strictly set down the same as PSR though.

Emergency Speed Restrictions - same as above

Junction speed restrictions - same as curvature of the track.

Al speed restrictions have a buffer so for example if the PSR is 50mph it will still be safe if a train is at 60mph this isn't the actual tollerance but gives you an idea.

You have differential speed restrictions as well for Freight & Light locos and these are lower than the line speed and also enhanced speed restrictions for certain types of traction for example the HST speeds are usually down to the brake force of the trains involved as are SP, MU, EPS & some others. There are also derrogations in the notices one I know of is class 67's being able to run at line speed because of their enhanced brake force where say a class 57 could not

Hope this is of help
 

Tomnick

Established Member
Joined
10 Jun 2005
Messages
5,390
Signal sighting (distant signals) - the speed is set with the sevice braking distace of the lowest brake force traction taken into account
Certainly the case for signal spacing, but surely braking distance is largely irrelevant in signal sighting? The need is to provide x seconds (8?) of uninterruputed sighting of the signal at linespeed, with the braking distance commencing at the signal itself (which is when signal spacing is considered).
Curvature of track - speed set below the advised speed that a train could derail
Correct - but interesting to note that the limiting factor is usually passenger comfort (and no doubt longer-term maintenance considerations), which becomes an issue at a much lower speed than a derailment risk!
 
Joined
6 Sep 2011
Messages
931
Location
Blackpool south Shore
Correct - but interesting to note that the limiting factor is usually passenger comfort (and no doubt longer-term maintenance considerations), which becomes an issue at a much lower speed than a derailment risk!
Agreed.
A 40mph curve could stand say 70mph, but the front wheels would tend to jolt fairly violently in the curve.
 

TDK

Established Member
Joined
19 Apr 2008
Messages
4,126
Location
Wrexham
Certainly the case for signal spacing, but surely braking distance is largely irrelevant in signal sighting? The need is to provide x seconds (8?) of uninterruputed sighting of the signal at linespeed, with the braking distance commencing at the signal itself (which is when signal spacing is considered).
If a distant signal is poorly sighted on a curve you will on occasions get a speed restriction in place where if the signal was well sighted the restriction would not be in place. This is only for distant signals and not very common these days
 

Tomnick

Established Member
Joined
10 Jun 2005
Messages
5,390
I agree. If it's for lack of sighting distance though, it's nothing to do with the braking distance from the signal itself! On the other hand, 35/50 speeds appeared at a couple of locations on the Skegness branch proper (east of Firsby, linseed generally 50mph) extending from the distant signal to the home signal. I can only assume that they were down to insufficient braking distance for the trains to which the lower speed applied!
 

Joseph_Locke

Established Member
Joined
14 Apr 2012
Messages
1,868
Location
Within earshot of trains passing the one and half
Agreed.
A 40mph curve could stand say 70mph, but the front wheels would tend to jolt fairly violently in the curve.

Two different things. Your jolt is cured by providing a transition curve from straight to curve and back again.

As Tomnick says, the limit case speed (at which the acceleration vector due to speed, curvature and gravity points outside the outside rail, and the train falls over) for a typical train is in excess of anything a passenger (and a passenger's hot beverage) will put up with. All a tilting train does is tilt the passenger even further that the track is already tilted, so that the passengers don't notice the increased lateral acceleration when cornering.

The limit for a normal train under perfect circumstances is a combined cant vale and "discomfort factor" (called cant deficiency in the business) of 300 mm. a tilting train can add another 150mm to this, making 450mm. However, the combined cant / cant deficiency factor goes up as the square of speed:

E+D = 11.82 * v^2 / R

so a 50% increase in cant / cant deficiency factor only allows a 20% increase in speed.

Form this you can deduce the maximum permissible lateral acceleration passengers are exposed to ...
 

Dan_

New Member
Joined
8 May 2012
Messages
1
Joseph has essentially defined the factors that, from a track point of view, limit speed; cant, cant deficiency and radius.

I don't know the site but assuming the (normal) maximum cant and cant deficiency values were applied and the curve was a tight radius of 200m then;

Cant, E, = 150mm
Cant deficiency, D, = 90mm (for jointed track)
Radius, R, = 200m
Max speed (kph), Vmax = ???

Vmax = square root((R x (E + D)/11.82)
=> Vmax = square root((200 x (150 + 90))/11.82)
=> Vmax = 64kph
=> Vmax ~ 39.5mph

This figure must be rounded down to the nearest 5mph, making the theoretical max speed 35mph.

It is not permitted to convert jointed track to continuously welded rail (CWR) at such tight radii, but if it were then your maximum can deficiency would increase to 110mm. Plugging 110 into the formula above would do just enough to be able to increase the speed to 40mph.

Rates of change of cant and cant deficiency are also import; you can't go from straight track with 0 cant to a 200m curve and 150mm cant instantaneously - this is where transitions come in. You need to gradually decrease the radius and increase the cant for when you are entering the curve and then the opposite when you're exiting it. Essentially, this makes the curve longer and takes up more room. Assuming there was room for these transitions and the rates of change were okay, the question is then why isn't the speed 35mph?

As others have said, track is only one of many factors to consider. Perhaps there is insufficient signal sighting time at such speed, or the additional speed/ballast (to produce the cant) would result in too much static/dynamic load on the bridge, or the train's 'kinematic envelope' (essentially how much it bounces around) would become to large to clear the bridge/other structures or trains and so on... My understanding is that you aim for the highest speed to begin with and then decrease until you're able to satisfy all site/engineering constraints.

Another possibility is that a higher speed is possible at that location, but either side of it only 15mph is achievable. Then it becomes a question of is it worth the extra money to get that higher speed over a short section? A higher running speed may also result in a requirement for an enhanced maintenance regime but if there's no operational advantage of that extra speed then it doesn't make sense to have it.


I should point out at this point that I am NOT an expert and what I've written is just my understanding based on my current knowledge but hope is was of some use/interest!
 

Hydro

Established Member
Joined
5 Mar 2007
Messages
2,204
Form this you can deduce the maximum permissible lateral acceleration passengers are exposed to ...
Having traversed zero cant curves at speeds up to and including 90mph, this part is actually very important. It's rather uncomfortable.
 

Joseph_Locke

Established Member
Joined
14 Apr 2012
Messages
1,868
Location
Within earshot of trains passing the one and half
Having traversed zero cant curves at speeds up to and including 90mph, this part is actually very important. It's rather uncomfortable.
... and it sloshes your coffee about. This analogy was the one I was taught when I learned about curving, at a very tender age.

However, since GCC and RCF "became" big issues, the boffins have now concluded that high cant deficiency on tight curves is a good thing, to the point where, on curves tighter than 1000m, it will be the new practice to only apply cant if you have to, e.g. maximise deficiency first.
 
Status
Not open for further replies.

Top