Why can HS2 go faster?

[ddebecker841]

I was thinking about this the other day and was hoping the forum could help me out. Why question seems quite simple but even with all my google searches I have no idea why the HS2 can go faster, the suggestions I have come up with include.

It may use a wide gauge track. Which I understand increases stability at high speed.
It uses a different rail profile/ wheel profile combo which is optimised for speed.
It uses better ballast thus reducing the chance of twist etc...
It uses better track materials for both the rail and/or sleepers

I understand they must be using better engines to get that higher top speed, but what stops us implementing this nation wide. Is it a physical limitation or just a monetary one. I also understand that congestion would be a problem specifically if there was fast and and 'slow' trains on the same line. I am specifically talking about phase 1 where they will be going from London to the West Midlands.

 

[55z]

In the UK the time saving between 270 kph and 300 kph on the London to Birmingham sector is around 2 minutes but energy consumption nearly doubles, from 300 kph to 320 kph saves around 2 minutes and again energy consumption increases substantially besides changing the overhead et etc.

 

[Domh245]

The main reasons that I can think of are signalling and the layout of the lines. HS2 will have in cab signalling which means that they can run trains faster than if they used colour light signals (which in the UK are limited to 125mph) as you can in essence give the train more notice of when it needs to stop whilst keeping reasonable braking distances. The other reason is that many lines aren't suitable for high speed running. The WCML is far too bendy (HS2 will be more straight) and the ECML doesn't have suitable infrastructure (the overhead lines being susceptible to fall down if someone sneezes in their direction - at high speeds they wouldn't fare well at all)

 

[AM9]

In the UK the time saving between 270 kph and 300 kph on the London to Birmingham sector is around 2 minutes but energy consumption nearly doubles, from 300 kph to 320 kph saves around 2 minutes and again energy consumption increases substantially besides changing the overhead et etc.

Can you please substantiate your assertion that I've bolded and italicised above? A valid hyperlink to your source could make the statement credible.

 

[AM9]

I was thinking about this the other day and was hoping the forum could help me out. Why question seems quite simple but even with all my google searches I have no idea why the HS2 can go faster, the suggestions I have come up with include.

It may use a wide gauge track. Which I understand increases stability at high speed.
It uses a different rail profile/ wheel profile combo which is optimised for speed.
It uses better ballast thus reducing the chance of twist etc...
It uses better track materials for both the rail and/or sleepers

I understand they must be using better engines to get that higher top speed, but what stops us implementing this nation wide. Is it a physical limitation or just a monetary one. I also understand that congestion would be a problem specifically if there was fast and and 'slow' trains on the same line. I am specifically talking about phase 1 where they will be going from London to the West Midlands.

It will have standard gauge track just like every other mainline railway (including HS1) in the UK and pretty well every other high speed railway in the world..

 

[pdeaves]

Non-'high speed' railways were laid out yonks ago for the fastest trains of the time. HS2 will be specially designed and built with faster trains in mind so will be straight enough to run faster trains without them falling over. Thus, in answer to your question there are physical limitations in running faster. A reasonably analogy would be twisty country lanes vs motorways. One is designed for high speeds and the other isn't.

 

[MarkyT]

The primary enabler of high speed is the track alignment which is much straighter than most historic lines. Quite steep gradients are permitted at full speed, up to 1:40, subject to vertical curvature limits. Tunnels need a larger cross sectional area for high speed. In places along HS2, bored tunnels will impose a limit on speed lower than the highest speed sections in open air and cut and cover green tunnels. The most significant bored tunnels are planned to be at the route's extremities, approaching London and Manchester where where trains will be accelerating away from the termini. Signalling in UK must be cab based for any speeds above 125 mph.

 

[edwin_m]

The tracks are spaced further apart. This allows the use of wider European-sized trains (though they won't be used initially) but more importantly reduces aerodynamic effects when trains pass each other and therefore allows them to do so at higher speeds. Stations are also designed with platforms on lower-speed loop tracks, as a train passing a platform at full speed would be dangerous to anyone standing there.

 

[MarkyT]

The tracks are spaced further apart. This allows the use of wider European-sized trains (though they won't be used initially) but more importantly reduces aerodynamic effects when trains pass each other and therefore allows them to do so at higher speeds.

Wider spacing also allows most forms of inspection and maintenance work to take place safely on one closed track while the other remains open to traffic.

 

[HSTEd]

It's mainly something that you can do if you design from the beginning to do it and accept the design constraints it provides.

Most of the existing WCML was original laid out for 30mph or so in the mid-19th Century.

 

[LNW-GW Joint]

It's mainly something that you can do if you design from the beginning to do it and accept the design constraints it provides.
Most of the existing WCML was original laid out for 30mph or so in the mid-19th Century.

In fact, it's something of a triumph of Georgian engineering that routes laid out in the mid-1830s are capable of 125mph today.

While alignment and signalling are key to the maximum speed, other factors play their part, including the strength of the track and structures, and the quality of the overhead line and power supply systems.
Classic lines are not capable of high speed in any of these areas.
The sheer complexity of the classic network, with frequent conflicting junctions and highly-variable train types and speeds, also prevents high speed operation.

 

[Ianno87]

Can you please substantiate your assertion that I've bolded and italicised above? A valid hyperlink to your source could make the statement credible.

To be fair, doing a simple Time = Distance / Speed calculation pretty much suffices for the first part...

(Birmingham-London is c. 180km, less accelleration + deceleration, plus the Birmingham Interchange stop)

 

[Randomer]

While alignment and signalling are key to the maximum speed, other factors play their part, including the strength of the track and structures, and the quality of the overhead line and power supply systems.
Classic lines are not capable of high speed in any of these areas.

Isn't part of the ECML set up for 140mph running, around Selby IIRC, including the power supply and signalling system (double flashing yellows for a third block before singal at danger comes to mind but not precise). Was it not then a policy decision by the BR board at the time not to run over 125mph without in cab signalling or a second man. Done during electrification in the early 1990's.

 

[edwin_m]

The flashing greens are on part of the section between Peterborough and Grantham (incidentally the site of Mallard's record run) but were only ever intended for testing the IC225 sets at their intended maximum speed of 140mph, as BR had to do that before accepting them from the builders. The Inspectorate was never going to allow passenger service at that speed without an ATP system.

The Selby Diversion was opened in about 1983, paid for by the Coal Board so they could mine underneath the old formation (fat lot of good that did them...) and incidentally routeing the ECML away from a very slow section round the curves and over the swing bridge at Selby. Therefore it's the newest part of the ECML and would probably be one of the section of 140mph running if that was ever to happen on the route. I don't think electrification had go that far north at the time the IC225s were delivered, so it couldn't have been used for testing them.

 

[gsnedders]

The Selby Diversion was opened in about 1983, paid for by the Coal Board so they could mine underneath the old formation (fat lot of good that did them...) and incidentally routeing the ECML away from a very slow section round the curves and over the swing bridge at Selby. Therefore it's the newest part of the ECML and would probably be one of the section of 140mph running if that was ever to happen on the route. I don't think electrification had go that far north at the time the IC225s were delivered, so it couldn't have been used for testing them.

Someone in another thread claimed a design speed of 160mph for the Selby Diversion, and it's definitely over 140mph.

 

[richieb1971]

I was thinking about this the other day and was hoping the forum could help me out. Why question seems quite simple but even with all my google searches I have no idea why the HS2 can go faster

Straight line track (Straight track is more important to speed than the power of the train)
Trains will likely be multi wheel/carriage drive (most trains in the UK on inter city routes are powered only by wheels on the ends of the train, or even just one end of the train).
Clearances
Lack of conventional things like railway crossings which keep top speeds down

 

[edwin_m]

Straight line track (Straight track is more important to speed than the power of the train)
Trains will likely be multi wheel/carriage drive (most trains in the UK on inter city routes are powered only by wheels on the ends of the train, or even just one end of the train).
Clearances
Lack of conventional things like railway crossings which keep top speeds down

Most multiple units have been 25% and 66% of axles motored. This helps with acceleration, but particularly for a high speed train power becomes very important as the air resistance goes up dramatically at higher speeds.

 

[HSTEd]

In Shinkansen it has reached the point where the N700 has 56 motored axles out of 64 total, and the 500 Series had every axle motored.

 

[radamfi]

Why will HS2 be able to go at 250 mph (400 km/h) when other high speed lines in the world only go at 300-320 km/h?

 

[takno]

Why will HS2 be able to go at 250 mph (400 km/h) when other high speed lines in the world only go at 300-320 km/h?

As I understand it that's the design speed. The normal running speed when trains aren't trying to recover from delays etc will be a fair bit closer to the 320 figure. The main reason other trains have settled on a 320km/h top speed even though several are capable of doing faster is that that is the speed where the extra speed stops being worth the rapidly increasing cost of power consumption. It's possible that we've decided that the acceptable power consumption and cost is higher in return for faster journeys, but I think mostly it's a combination with wanting to be able to run faster to get things back on schedule, and being prepared if something comes along which alters the power costs substantially in the future.

 

[DaveNewcastle]

Someone in another thread claimed a design speed of 160mph for the Selby Diversion, and it's definitely over 140mph.

Really? I find that 125 over the Hambleton Junctions is uncomfortable. So for the 5 miles between Temple Hirst and Hambleton Sth., and the 8 miles between Hambleton Nth. and Colton, it's hardly worth attempting anything faster.

 

[Maurice3000]

Why will HS2 be able to go at 250 mph (400 km/h) when other high speed lines in the world only go at 300-320 km/h?

The French LGV Est has a design speed of 350 km/h. That is the track where they set the record speed of 574.8 km/h in 2007 (a record attempt that required specific adjustments to train and infrastructure).

At 400 km/h HS2 will have a 50 km/h higher design speed, mainly because you design these alignments for a century or more so you might as well plan for some margin. In practice I don't see the first trains to run on it to have top speeds over 320 km/h. The economics don't make sense for higher speeds, yet. That might be quite different in 2040...

 

[radamfi]

The French LGV Est has a design speed of 350 km/h. That is the track where they set the record speed of 574.8 km/h in 2007 (a record attempt that required specific adjustments to train and infrastructure).

At 400 km/h HS2 will have a 50 km/h higher design speed, mainly because you design these alignments for a century or more so you might as well plan for some margin. In practice I don't see the first trains to run on it to have top speeds over 320 km/h. The economics don't make sense for higher speeds, yet. That might be quite different in 2040...

Does the 18 trains per hour frequency depend on trains running at 400 km/h?

 

[edwin_m]

Does the 18 trains per hour frequency depend on trains running at 400 km/h?

No. The timetables assume a maximum of 360km/h.

At higher speeds (above a certain speed that's a lot less than 360km/h) capacity is reduced. The time interval between trains depends on how long they take to cover the distance separating them. If this distance was constant then the time interval would reduce proportionately to train speed. However the separation between the trains depends on the braking distance (amongst other things) which increases proportionately to the square of train speed.

So the 18TPH might actually increase at lower speeds, if there were enough platforms at Euston to turn them all round.

 

[ForTheLoveOf]

Can you please substantiate your assertion that I've bolded and italicised above? A valid hyperlink to your source could make the statement credible.

The simple kinetic energy equation: K.E. = 1/2×m×v^2 shows that at 320km/h, assuming constant m, the energy requirement is approximately 14% higher than at 300km/h. Considering that wind resistance is also a square function of the speed (velocity), this too is approximately 14% higher at 320km/h.

So whilst twice the energy is probably an overstatement, it would not be unreasonable to think that a 50% or greater increase in input energy would be required to attain 320km/h.

 

[edwin_m]

The simple kinetic energy equation: K.E. = 1/2×m×v^2 shows that at 320km/h, assuming constant m, the energy requirement is approximately 14% higher than at 300km/h. Considering that wind resistance is also a square function of the speed (velocity), this too is approximately 14% higher at 320km/h.

So whilst twice the energy is probably an overstatement, it would not be unreasonable to think that a 50% or greater increase in input energy would be required to attain 320km/h.

However you get the kinetic energy back via regenerative braking when slowing down (and a benefit of having a lot of axles motored is that a large proportion of kinetic energy is recovered - especially if the braking rate is relatively modest). It's only really the resistance losses that are relevant to energy consumption at a particular speed. Rolling resistance goes up approximately linearly and aerodynamic resistance does indeed go up as the square, although there may be other factors coming into play at that sort of speed.

 

[ForTheLoveOf]

However you get the kinetic energy back via regenerative braking when slowing down (and a benefit of having a lot of axles motored is that a large proportion of kinetic energy is recovered - especially if the braking rate is relatively modest). It's only really the resistance losses that are relevant to energy consumption at a particular speed. Rolling resistance goes up approximately linearly and aerodynamic resistance does indeed go up as the square, although there may be other factors coming into play at that sort of speed.

Indeed - I don't think it's the pure kinetic energy and 'natural' wind resistance that play the main roles here - it will be issues like additional energy loss from friction, as well as the interaction of wind/air waves with tunnels, cuttings, other trains etc. (Bearing in mind you would be passing the equivalent of 36tph relative to the opposite-direction line).

 

[AM9]

However you get the kinetic energy back via regenerative braking when slowing down (and a benefit of having a lot of axles motored is that a large proportion of kinetic energy is recovered - especially if the braking rate is relatively modest). It's only really the resistance losses that are relevant to energy consumption at a particular speed. Rolling resistance goes up approximately linearly and aerodynamic resistance does indeed go up as the square, although there may be other factors coming into play at that sort of speed.

In addition, much of the kinetic energy invested at the higher speed will aid the speed stability of the train through inclines en route , - a feature that detractors of high speed rail seem to ignore. Even drags such as Shap or Beattock are effectvely smoothed out when travelling at high speed making the maintenance of constant headways easier.

 

[edwin_m]

In addition, much of the kinetic energy invested at the higher speed will aid the speed stability of the train through inclines en route , - a feature that detractors of high speed rail seem to ignore. Even drags such as Shap or Beattock are effectvely smoothed out when travelling at high speed making the maintenance of constant headways easier.

In fact as I wrote the previous post I was thinking of the original LGV to Lyon, which is a real switchback. The trains run below their permitted speed on average, and this allows them to speed up and slow down on the hills without constantly motoring and braking to stay within the limit. I think they run closer to maximum speed if they are late. HS2 mostly doesn't encounter that sort of undulating terrain so there will be less need for that sort of approach. However ascending a gradient does need more power at high speed, because the weight of the train is lifted more quickly. Applying the published tractive effort curve of the Alstom AGV to the published gradient profile of HS2 phase 1 shows that it can't maintain maximum speed on the long climb to the Chilterns. All trains are affected equally so the capacity penalty is small.

 

[Wychwood93]

The simple kinetic energy equation: K.E. = 1/2×m×v^2 shows that at 320km/h, assuming constant m, the energy requirement is approximately 14% higher than at 300km/h. Considering that wind resistance is also a square function of the speed (velocity), this too is approximately 14% higher at 320km/h.

So whilst twice the energy is probably an overstatement, it would not be unreasonable to think that a 50% or greater increase in input energy would be required to attain 320km/h.

That is how physics works - the laws of same are immutable - regardless of DfT notions. I still struggle with the 18 trains p.hour to Brum - why? To have that capacity is fine but, when passenger figures have started to drift off in the SE..... Could there really be a max. demand of some 18k people p.hour to Brum and back?