Railperf
Established Member
- Joined
- 30 Oct 2017
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Using a claimed 'acceleration rate is misleading, as no train maintains a fixed acceleration rate across the speed range. Probably the class 91's seem to have the most linear speed vs time graph up to 100mph compared to other stock.
And two trains that appear to accelerate in the same time to a specific speed - say 60mph - can display different characteristics in achieving that speed.
One of the issues overlooked by many - are the effects of low-speed restrictions over junctions and turnouts into station platforms. A 2 or 3 car train can clear the low-speed restriction and start accelerating much earlier. A much longer - say 12 car - train may have superior acceleration but the driver has to wait until the rear of the train clears the speed restriction. - By which time the slower accelerating train is already off in the distance.
That is why 2-car 158's set faster station to station times than HST's between certain stations in Cornwall and Scotland.
I've recently been comparing in close detail - the performance of Class 90's vs HST's and 91's on the ECML stopping services. Taking Peterborough to Grantham as an example - a Class 90's superior low speed acceleration to 110mph puts it far enough ahead of both a Class 91 and an HST, that it takes the HST and the 91 almost until Stoke Tunnel from a Peterborough start to be level pegging with the 91. And at that point the speed limit drops to 115mph , so quite astonishingly, the 90 can achieve an almost identical station to station timing despite a 15mph deficit in top speed.
On a route with differing speed limits and distances between station stops, you have to look at how each type of traction will fare on each section. But sometimes even that is irrelevant. Station dwell times / passenger loading and unloading can make a bigger difference during busy times than good acceleration! Although people hate the former london Underground D stock, there has barely been a better case for what i witnessed as a slick station dwell operation. The doors would open instantly as soon as the train had reached the station. and from the moment the doors had closed, the train was under power and accelerating away. Class 378s in comparison seem to suffer a delay between the train stopping and the doors opening, and a similar delay between the doors closing and train actually starting to move. From a passenger perspective the 378, despite being a faster accelerating train seemed slower along the route!
Timetabling is another issue. the current GWR timetable shows what happens when too much slack is in the timetable. Trains leaving London arrive at Reading too early and cannot get into the station because the platforms are still occupied by trains that have not yet reached their planned departure slot.
All the customer knows, is that he is sitting in the middle of nowhere wondering why his train has stopped - and maybe starting to panic as to whether there is a problem.
Having recently spent a lot of time at Peterborough and Doncaster recently, i saw the effect of not having enough platforms, plus the problems caused by there being too many flat junctions. The Sheffield services at Doncaster have to cross the down main line from Platform 3 - usually causing a down fast train to be sitting outside the station waiting for the stopper to cross over and the junction to clear. how many times have i been on a down train crawling into Doncaster? And when i check the open train times map - there is a train sitting ahead of us in the platform!
At Peterborough trains coming into the up main platform seem to always end up arriving on cautionary signals - which slows down the station to station time - without there seeming to be any conflicting traffic around.
Operationally - it would make sense to allow stopping trains should be able to approach the platform at the same speeds as if the platform starter was at green. That would seemingly speed up journeys and cause less delay! A more advanced safety system such as the French KVB would probably be required to supervise the braking curve. I guess we will have to wait for ECTS.
And two trains that appear to accelerate in the same time to a specific speed - say 60mph - can display different characteristics in achieving that speed.
One of the issues overlooked by many - are the effects of low-speed restrictions over junctions and turnouts into station platforms. A 2 or 3 car train can clear the low-speed restriction and start accelerating much earlier. A much longer - say 12 car - train may have superior acceleration but the driver has to wait until the rear of the train clears the speed restriction. - By which time the slower accelerating train is already off in the distance.
That is why 2-car 158's set faster station to station times than HST's between certain stations in Cornwall and Scotland.
I've recently been comparing in close detail - the performance of Class 90's vs HST's and 91's on the ECML stopping services. Taking Peterborough to Grantham as an example - a Class 90's superior low speed acceleration to 110mph puts it far enough ahead of both a Class 91 and an HST, that it takes the HST and the 91 almost until Stoke Tunnel from a Peterborough start to be level pegging with the 91. And at that point the speed limit drops to 115mph , so quite astonishingly, the 90 can achieve an almost identical station to station timing despite a 15mph deficit in top speed.
On a route with differing speed limits and distances between station stops, you have to look at how each type of traction will fare on each section. But sometimes even that is irrelevant. Station dwell times / passenger loading and unloading can make a bigger difference during busy times than good acceleration! Although people hate the former london Underground D stock, there has barely been a better case for what i witnessed as a slick station dwell operation. The doors would open instantly as soon as the train had reached the station. and from the moment the doors had closed, the train was under power and accelerating away. Class 378s in comparison seem to suffer a delay between the train stopping and the doors opening, and a similar delay between the doors closing and train actually starting to move. From a passenger perspective the 378, despite being a faster accelerating train seemed slower along the route!
Timetabling is another issue. the current GWR timetable shows what happens when too much slack is in the timetable. Trains leaving London arrive at Reading too early and cannot get into the station because the platforms are still occupied by trains that have not yet reached their planned departure slot.
All the customer knows, is that he is sitting in the middle of nowhere wondering why his train has stopped - and maybe starting to panic as to whether there is a problem.
Having recently spent a lot of time at Peterborough and Doncaster recently, i saw the effect of not having enough platforms, plus the problems caused by there being too many flat junctions. The Sheffield services at Doncaster have to cross the down main line from Platform 3 - usually causing a down fast train to be sitting outside the station waiting for the stopper to cross over and the junction to clear. how many times have i been on a down train crawling into Doncaster? And when i check the open train times map - there is a train sitting ahead of us in the platform!
At Peterborough trains coming into the up main platform seem to always end up arriving on cautionary signals - which slows down the station to station time - without there seeming to be any conflicting traffic around.
Operationally - it would make sense to allow stopping trains should be able to approach the platform at the same speeds as if the platform starter was at green. That would seemingly speed up journeys and cause less delay! A more advanced safety system such as the French KVB would probably be required to supervise the braking curve. I guess we will have to wait for ECTS.
