Why not 125mph?

[alexl92]

Forgive me if I'm mistaken but to my knowledge all recent orders of DMUs, some EMUs and even the Class 68s & 88s hvae been spec'd for 100 or 110mph running only.

During discussions about future cascades on this forum, units are frequently discussed for a transfer to a different area, only for someone to point out that said unit can 'only' do 100, or 110, meaning that they'd cause capacity issues - an example being the Class 90s being used to help out on ECML services.

Why aren't more units & locos designed for 125mph?
I've heard the argument that in some places we don't need DMUs that can do it because all the 125mph lines will be electrified soon - but that doesn't take into account routes where the train travels on a mixture of routes for a service - such as the TPE 185s when they run on the ECML.

Equally, why are the 68s and 88s only 100mph engines? Couldn't the ability for the 88s to do 125 under the wires allow them to cover for 91s on the ECML without the issues that come with using a 110mph Class 90? And surely even 110mph would be of more benefit should the 68s ever be used as Thunderbirds or to haul LHCS passenger trains on the big mainlines?

I'm sure there's something big that I've missed about this, but I'm just interested to know. It always seems like short-sighted thinking, given the rate at which the railway is striving to increase capacity as much as possible.

 

[cjmillsnun]

Forgive me if I'm mistaken but to my knowledge all recent orders of DMUs, some EMUs and even the Class 68s & 88s hvae been spec'd for 100 or 110mph running only.

During discussions about future cascades on this forum, units are frequently discussed for a transfer to a different area, only for someone to point out that said unit can 'only' do 100, or 110, meaning that they'd cause capacity issues - an example being the Class 90s being used to help out on ECML services.

Why aren't more units & locos designed for 125mph?
I've heard the argument that in some places we don't need DMUs that can do it because all the 125mph lines will be electrified soon - but that doesn't take into account routes where the train travels on a mixture of routes for a service - such as the TPE 185s when they run on the ECML.

Equally, why are the 68s and 88s only 100mph engines? Couldn't the ability for the 88s to do 125 under the wires allow them to cover for 91s on the ECML without the issues that come with using a 110mph Class 90? And surely even 110mph would be of more benefit should the 68s ever be used as Thunderbirds or to haul LHCS passenger trains on the big mainlines?

I'm sure there's something big that I've missed about this, but I'm just interested to know. It always seems like short-sighted thinking, given the rate at which the railway is striving to increase capacity as much as possible.

We already have 125 capable locos that sit around idle (67s) and there will be spare 91s soon (once the 800/801s are in service on the ECML). Is it worth spending the extra money to increase the speed of the 68s and 88s and reduce their acceleration as well as reduce their tractive effort? Doing so will mean that they aren't suited for any freight use.

 

[tbtc]

It's a trade off between top speed and acceleration. Think back to Mario Kart days.

Most freight involves a fair bit of stopping/ starting (sometimes due to being looped) and getting from 0mph to 50mph is more important than getting above 50mph - with very little chance of getting a heavy freight train above 100mph.

With DMUs/ EMUs, you have the "crumple zone" issue - fine to run at 125mph when loco hauled (and passengers aren't in the leading vehicle), but you have to give up a lot of space at the front of a DMU/EMU as a "crumple zone" if you want it to run at 125mph (which also means giving up an equivalent amount of space in the rear vehicle). Hence why the front carriages of Voyagers have so few seats in them.

Plus, with DMUs on stop/start routes, acceleration is more important than the time saved by going at 125mph against 100mph over a few miles (you'll need to get from 0mph to 50mph several times a journey).

 

[CosherB]

We already have 125 capable locos that sit around idle (67s) and there will be spare 91s soon (once the 800/801s are in service on the ECML). Is it worth spending the extra money to increase the speed of the 68s and 88s and reduce their acceleration as well as reduce their tractive effort? Doing so will mean that they aren't suited for any freight use.

It would make the Class 67s far more attractive if they were re-geared for a top speed of 100mph. Not that it will happen ....

 

[bastien]

I'd imagine the beating a ~85 ton loco gives the track at 125 mph ups the charges, too, compared to a 50 ton multiple unit vehicle.

 

[hurricanemk1c]

Different crash worthiness standards applies to 125 mph, where you have to have no passenger accommodation in the front 1/3 of the leading vehicles, thus loosing quite a bit of capacity. Plus the stuff above about gearing etc.

The rules changed a couple of years ago from 160 km/h to 185 km/h (I think), which to the closest round number is 110mph (think the limit is actually something like 112.5)

 

[snowball]

The rules changed a couple of years ago from 160 km/h to 185 km/h (I think), which to the closest round number is 110mph (think the limit is actually something like 112.5)

185 km/h is 114.95 mph, so with a tiny bit of generosity you could allow 115.

If the limit was 180 km/h, that would be 111.85.

A limit that's a multiple of 10 in km/h sounds slightly more plausible.

 

[najaB]

I'm sure there's something big that I've missed about this, but I'm just interested to know. It always seems like short-sighted thinking, given the rate at which the railway is striving to increase capacity as much as possible.

Top speed, tractive effort/acceleration, cost (both to build and to operate). Choose which two you want to optimise at the expense of the third.

 

[55z]

Good points about current safety regs and gearing AND Trans Pennine & Hull trains have both ordered 125 mph electro/diesel trains and GWR have increased their order for 125 mph trains.

 

[Envoy]

Well, if they are so concerned about safety and crumple zones, what about drivers in the confined cabs of 90mph 158’s - with the passengers not far behind?

 

[najaB]

Well, if they are so concerned about safety and crumple zones, what about drivers in the confined cabs of 90mph 158’s - with the passengers not far behind?

I could be wrong, but by my reckoning 90mph is less than 100mph. So different rules apply.

 

[edwin_m]

I could be wrong, but by my reckoning 90mph is less than 100mph. So different rules apply.

The rules have also changed in recent years to incorporate some energy absorption material in the front end, explaining why the noses of recent units are longer than older designs. This would make little difference in the event of a head-on train collision at full speed, but might be significant in a lower-speed or particularly a level crossing impact.

 

[NSEFAN]

The rules have also changed in recent years to incorporate some energy absorption material in the front end, explaining why the noses of recent units are longer than older designs. This would make little difference in the event of a head-on train collision at full speed, but might be significant in a lower-speed or particularly a level crossing impact.

Yes. At the moment, level crossing incidents are more likely than head-on collisions, thanks to TPWS.