Future of HSR - Lighter trains? More wheels?

[bmbm]

Hi,

Lately I got very interested in highspeed trains. I was wondering what the future looks like for the regular HSR, wheels on rails, not the Maglev variant. My interest was more in weight reduction, and improving traction.

The Shinkansen N700's passenger car is 25m long, has 2 sets of 4 wheels each and weighs 46 tons.

That is:
1.84 tons/m.
5.75 tons/wheel

In theory, if budget would not be an issue, how much lighter can a train become such as the Shinkansen N700 before it starts to lose traction?
And when it would start to lose traction - because not enough weight would be pushing down on the wheels - would it help to add wheels (larger surface in contact with rails) and improve stability (catterpillar effect)?

Adding wheels reduces weight/wheel, but adds more contact to the rails. So what is it?

It's been bugging me for over a week, and I can't figure it out .

Edit: To keep it simple, I kept aerodynamics out of it. Also, to keep things fun, engine power would be unlimited.

 

[squizzler]

Typically raifways talk about mass per axle rather than per wheel. Your example is thus 11.5 tons (sic) - I imagine that will be metric. European standards allow a maximum of 17 tonnes (metric ton), following the French experience with their TGVs. I don't think losing traction is a concern because all Shinkansen axles are driven.

As I understand it, fewer axles are desirable because there are fewer moving parts to maintain and wheels to put through the wheel lathe. So most European HSTs choose to be closer to the permitted maximum than the Japanese example.

 

[edwin_m]

This is all about tractive effort, which is the force that the wheels can provide to accelerate the train.

The amount of tractive effort possible from one powered axle depends on the weight on that axle and the coefficient of friction between the wheel and the rail. So if you keep the number of powered axles and the coefficient of friction the same, but reduce the weight, then this also reduces the maximum tractive effort possible before the wheels lose adhesion and just spin. But a lighter train also needs less tractive effort to accelerate it and, if you ignore aerodynamic and other resistances to motion, these effects cancel out so reducing the weight doesn't affect the acceleration at all! A lighter train does however need less energy to accelerate it.

In practice the rolling resistance will pretty much vary with weight too, but aerodynamic resistance will stay the same (assuming same train size and shape). So a bigger proportion of the (reduced) tractive effort is needed to overcome air resistance and the acceleration will reduce. However, because the motors are producing less tractive effort, they need less power and you can probably add in more of them until all the axles have motors.

Increasing the size of the wheel would indeed increase the contact area with the rail, but it has virtually no effect on the tractive effort. In practice it would be better to make the wheels smaller, because they are then lighter and also have less rotational inertia. But if they get too small the increased contact forces may start permanently distorting the head of the rail (plastic deformation), small wheels may not be compatible with normal pointwork and there may be problems fitting big enough brake discs.

Adding more wheels doesn't help, because each wheel then has less weight on it, so can provide less tractive effort, and again the total acceleration doesn't change. More wheels would make it possible to add more motors, but it's easy enough to design motor systems to give the maximum tractive effort that is possible at normal levels of friction, so in practice adding more wheels would just increase the weight of the train and therefore the amount of energy it uses.

A lighter train would be less stable because there's less weight to hold it down against wind or other forces that might be trying to tip it over. But I think weight would have to reduce a lot before this became a significant problem.

 

[dm1]

The reason most high speed trains only run at around 300km/h is not technical, but economic. As the TGV speed record runs showed, it is perfectly feasible to run existing conventional high speed trains at much higher speeds with appropriate maintenance regimes and alignments. But the energy required to do so becomes so great, that it is no longer economically viable. Maintenance tolerances also become too fine.

The extra speed is no longer worth it. Particularly since threading a higher speed alignment in built-up or topographically difficult areas becomes extremely challenging, requiring extremely large curve radii. Even 300 km/h will not be possible for many parts of HS2 for example. On the sections where it is, the time saved by going faster will be limited.

 

[MattRat]

Do linear induction motor trains count? Since they still rest on wheels?

 

[edwin_m]

Do linear induction motor trains count? Since they still rest on wheels?

Linear induction means that train performance is no longer dependent on wheel-rail friction (although unless it was failsafe when braking, brakes and therefore the headway between following trains would still be adhesion dependent). That means that greater acceleration might be possible. However accelerations much greater than those at present would be a danger to anyone standing or walking in the train, so we'd probably need seatbelts for all passengers and many more on-train staff to enforce their wearing. Linear induction also requires a continuous metal "reaction plate" to be laid along the track.

 

[bmbm]

This is all about tractive effort, which is the force that the wheels can provide to accelerate the train.

The amount of tractive effort possible from one powered axle depends on the weight on that axle and the coefficient of friction between the wheel and the rail. So if you keep the number of powered axles and the coefficient of friction the same, but reduce the weight, then this also reduces the maximum tractive effort possible before the wheels lose adhesion and just spin. But a lighter train also needs less tractive effort to accelerate it and, if you ignore aerodynamic and other resistances to motion, these effects cancel out so reducing the weight doesn't affect the acceleration at all! A lighter train does however need less energy to accelerate it.

In practice the rolling resistance will pretty much vary with weight too, but aerodynamic resistance will stay the same (assuming same train size and shape). So a bigger proportion of the (reduced) tractive effort is needed to overcome air resistance and the acceleration will reduce. However, because the motors are producing less tractive effort, they need less power and you can probably add in more of them until all the axles have motors.

Increasing the size of the wheel would indeed increase the contact area with the rail, but it has virtually no effect on the tractive effort. In practice it would be better to make the wheels smaller, because they are then lighter and also have less rotational inertia. But if they get too small the increased contact forces may start permanently distorting the head of the rail (plastic deformation), small wheels may not be compatible with normal pointwork and there may be problems fitting big enough brake discs.

Adding more wheels doesn't help, because each wheel then has less weight on it, so can provide less tractive effort, and again the total acceleration doesn't change. More wheels would make it possible to add more motors, but it's easy enough to design motor systems to give the maximum tractive effort that is possible at normal levels of friction, so in practice adding more wheels would just increase the weight of the train and therefore the amount of energy it uses.

A lighter train would be less stable because there's less weight to hold it down against wind or other forces that might be trying to tip it over. But I think weight would have to reduce a lot before this became a significant problem.

This explains it. Thank you!

I actually thought that on the Shinkansen all axels were powered.
The benefit of more wheels could result in less weight per wheel, resulting in much less wear on the track.

The reason most high speed trains only run at around 300km/h is not technical, but economic. As the TGV speed record runs showed, it is perfectly feasible to run existing conventional high speed trains at much higher speeds with appropriate maintenance regimes and alignments. But the energy required to do so becomes so great, that it is no longer economically viable. Maintenance tolerances also become too fine.

The extra speed is no longer worth it. Particularly since threading a higher speed alignment in built-up or topographically difficult areas becomes extremely challenging, requiring extremely large curve radii. Even 300 km/h will not be possible for many parts of HS2 for example. On the sections where it is, the time saved by going faster will be limited.

Interesting to see it's more about economic reasons.
The TGV record is the reason I don't understand the effort of the Japanese to move to Maglev technology.

== Doublepost prevention - post automatically merged: 9 Jan 2022 ==

== Doublepost prevention - post automatically merged: 9 Jan 2022 ==

Do linear induction motor trains count? Since they still rest on wheels?

When I search for linear motors on trains I only find Maglev. So I don't think that's the same as wheel-powered trains.

== Doublepost prevention - post automatically merged: 9 Jan 2022 ==

Linear induction means that train performance is no longer dependent on wheel-rail friction

 

[MattRat]

When I search for linear motors on trains I only find Maglev. So I don't think that's the same as wheel-powered trains.

It seems to be pretty rare, and no one has bothered to give it an easily identifiable name yet like Maglev has. Basically, it has a third rail that works like Maglev to pull the train along and slow it down, but still rests on wheels on the other 2 tracks.

 

[bmbm]

It seems to be pretty rare, and no one has bothered to give it an easily identifiable name yet like Maglev has. Basically, it has a third rail that works like Maglev to pull the train along and slow it down, but still rests on wheels on the other 2 tracks.

Ah okay I thought you meant the Maglev. I will look if I can find anything about it.

 

[edwin_m]

The short Scarborough metro line in the Toronto suburbs was built for linear induction motors with conventional track (so not Maglev), and the same technology was used for the initial routes of the Vancouver Skytrain starting in around 1986. Later Skytrain routes have used conventional motorised wheels. I'm not aware of any non-Maglev high speed route using linear induction.