Tram Trains and their top speed

[HSTEd]

Not sure if this should be in this forum or rolling stock.

But does anyone know if there is any particular hard reason that tram-trains tend to be built with 100kph / 60mph top speeds?
Is there some change to the crash standards above this that makes 70/75 impractical, or is that the European railways where they were developed have little use for such speeds on their countryside sections?

I am wondering this because many of the places where tram trains might be useful (railways near Nottingham etc) have sections where speeds higher than 60mph are common and they might be able to significantly reduce travel times by operating at the higher speed.

After all going from 60mph to 75mph potentially reduces travel times by a quarter!

It can't be a power supply issue as heavy rail units with similar installed power reach higher speeds, so why is it? (A Citadis Dualis has up to 900kW of traction motor outputs, which makes it more powerful than a Class 317 at half the weight!)

 

[30907]

Not sure, but two possible factors are:
wheel profile might make higher speeds unsafe;
the need to have high rates of acceleration from 0-50 km/h for frequent stops in town* with the inevitable tradeoff of less oomph at higher speed.
*actually, a lot of tram-train routes have frequent stops between the historic stations out of town as well.

 

[edwin_m]

A fairly informed guess but not a definitive answer:

BR Research discovered in the 1960s that a rail bogie will oscillate from side to side ("hunting") above a certain critical speed - the train is effectively zigzagging along by bouncing back off flanges on alternate sides. The solution usually adopted to allow smooth running at higher speeds is to fit dampers to resist the rapid rotation of the bogie during hunting, but still allow the bogie to rotate more slowly to follow the track on curves. These can be seen on most of our faster rolling stock, as a shock absorber mounted lengthways along the side of the bogie with one end attached to the body via a bracket and the other end to the bogie itself. Significantly 75mph units like the 150 and 156 don't have these, but the 100mph class 170 does have dampers on the same basic design of bogie.

Tram-trains have to cope with curve radii down to 25m or less (Nottingham has an 18m curve), so the bogie has to turn to a much greater angle than on a train. This probably means that a railway-type damper couldn't be fitted as the amount of extension would be too great. Also, on entry/exit of a tight curve the bogie has to turn much more quickly than on a train, and resisting this motion would increase the risk of derailment due to flange climbing. The short wheelbase of a tram-train bogie may also reduce the critical speed when hunting sets in.

I recall some years ago taking a Karlsruhe tram-train to Rastatt (I think), on a route that was not officially high speed but did include some relatively fast running by suitable rolling stock including ICEs. The tram-train did seem very slow on this journey and hunting was very obvious.

 

[HSTEd]

Those sorts of problems sound like they are more in the "this is difficult and noone has a use case that currently justifies the expense" more than "it is impossible".

I assume you would have to engineer a far more sophisticated damper than the traditional passive railway type, and that could easily cost a large sum of money. For something that is not currently critical.

 

[baz962]

It

Not sure if this should be in this forum or rolling stock.

But does anyone know if there is any particular hard reason that tram-trains tend to be built with 100kph / 60mph top speeds?
Is there some change to the crash standards above this that makes 70/75 impractical, or is that the European railways where they were developed have little use for such speeds on their countryside sections?

I am wondering this because many of the places where tram trains might be useful (railways near Nottingham etc) have sections where speeds higher than 60mph are common and they might be able to significantly reduce travel times by operating at the higher speed.

After all going from 60mph to 75mph potentially reduces travel times by a quarter!

It can't be a power supply issue as heavy rail units with similar installed power reach higher speeds, so why is it? (A Citadis Dualis has up to 900kW of traction motor outputs, which makes it more powerful than a Class 317 at half the weight!)

It wouldn't be reduced by a quarter, it's a fifth. At 75 mph you would do 75 miles in one hour. At 60mph it would take an hour and 15 or 75 minutes. One hour or 60 is 4/5 Of 75. Your math is a little off buddy. Sorry to be pedantic.

 

[Chester1]

I am not sure there is much of a need for tram trains travelling over 60mph. Most proposed tram train routes have very frequent stops and would not have sufficient distance between stops to go much faster than 60mph. A modern tram train would beat sprinters on most commuter stopping services.

 

[HSTEd]

I am not sure there is much of a need for tram trains travelling over 60mph. Most proposed tram train routes have very frequent stops and would not have sufficient distance between stops to go much faster than 60mph. A modern tram train would beat sprinters on most commuter stopping services.

The thing is the UK has a lot of routes that would benefit from going into town centres but are interuban in nature.

For example the Nottingham-Lincoln line has large stretches of line speed above 60mph, and would not benefit from that many additional stops, and given the insane acceleration tram style trains are capable of 75mph does become somewhat plausible.

It also simplifies the conversion of lines to tram train services because it allows them to take on longer distance "residual" services that are really awkward to provide otherwise without shuttles and such things that drive away custom.