ERTMS and slip trains

[172007]

Always wondered whether extra paths could be won by using slip trains.

Two 5 cars leave Paddington. West of Swindon the rear set with a driver. The trains two trains are in communication via the signaling system with each other. The front set uncoupling, and slows down for the Bath line whilst the front set continues for Badminton and Cardiff. Would use one path vs 2 and save a while load of time.

The key is in cab signaling and both trains in coms with each other so if one emergency brakes the other does and if the retardation / speed reduction is not sufficient with the rear set then the front eases off the brakes to maintain a safe distance. It would require a couple of miles of concrete slab track to ensure perfectly level and straight to provide reliable uncoupling.

Seems a futuristic idea but is it technically possible.

Sure there are many places in the network where this may work and increase capacity.

 

[GW43125]

I'm fairly sure one of the draft timetables for the Great Western involved an awful lot of splitting/joining at places like Swindon and Oxford. Thankfully, it never saw the light of day as it's a performance nightmare. What exactly would the advantage be of this over splitting at Swindon (where both trains stop anyway)? Also slip coaches only work in one direction.

There's a reason we binned slip coaches 50+ years ago.

 

[172007]

I'm fairly sure one of the draft timetables for the Great Western involved an awful lot of splitting/joining at places like Swindon and Oxford. Thankfully, it never saw the light of day as it's a performance nightmare. What exactly would the advantage be of this over splitting at Swindon (where both trains stop anyway)? Also slip coaches only work in one direction.

There's a reason we binned slip coaches 50+ years ago.

Time save as both moving at 100mph+ whilst splitting and 1 path rather than 2. If done at Swindon one would lose a few mins whilst the other departed. Better examples are probably the WCML where a train splitting on the move before Rugby would use 1 path out of Euston to Rugby than 2 paths for trains going Trent Valley and Cov.

We got rid of slip coaches for a reason as the technology was not available for it to be safe. I am suggesting that technology has moved on and asking the question is it now safe and possible. Is their an advantage to it on our busy railways to combine trains of different destinations and not lose time through uncoupling on a station bit at speed.

 

[Nick Ashwell]

Time save as both moving at 100mph+ whilst splitting and 1 path rather than 2. If done at Swindon one would lose a few mins whilst the other departed. Better examples are probably the WCML where a train splitting on the move before Rugby would use 1 path out of Euston to Rugby than 2 paths for trains going Trent Valley and Cov.

We got rid of slip coaches for a reason as the technology was not available for it to be safe. I am suggesting that technology has moved on and asking the question is it now safe and possible. Is their an advantage to it on our busy railways to combine trains of different destinations and not lose time through uncoupling on a station bit at speed.

You said the first line, which is clearly unsafe and with huge risks, and then said we got rid of it due to past safety issues.

Safety is stronger now for good reason. What happens is the rear train has a control fault and speeds up fast than the front train and they collide? Or if they're too close for the points to change?

This is completely different to traditional slip coaches that stopped, and frankly completely unsafe

 

[The Planner]

Time save as both moving at 100mph+ whilst splitting and 1 path rather than 2. If done at Swindon one would lose a few mins whilst the other departed. Better examples are probably the WCML where a train splitting on the move before Rugby would use 1 path out of Euston to Rugby than 2 paths for trains going Trent Valley and Cov.

We got rid of slip coaches for a reason as the technology was not available for it to be safe. I am suggesting that technology has moved on and asking the question is it now safe and possible. Is their an advantage to it on our busy railways to combine trains of different destinations and not lose time through uncoupling on a station bit at speed.

How are you dealing with two trains in the same signalling section even with ETCS? We arent at moving block yet and level 2 is still one train between marker boards.

 

[GW43125]

How are you dealing with two trains in the same signalling section even with ETCS? We arent at moving block yet and level 2 is still one train between marker boards.

And to add to that, how do you get the trains far enough apart, in the short section between Swindon and Wooton Bassett, for the second train not to be slowing to a crawl waiting for the route? By which point, may as well just split at Swindon.

Also, nobody's considered how you put the trains back together again.

 

[Bald Rick]

It‘s still two paths approaching the ‘split’, as Train 2 would have to be slowed sufficiently in advance of the junction to enable the signalling system to detect that Train 1 had passed, was complete, swing the points, confirm the route for Train 2 is clear, and clear the signal / issue movement authority, before Train 2 had reached braking distance for the junction signal / marker board.

In reality that means there is no gain in time vs splitting when stationary.

 

[DerekC]

Coventry University and Interfleet carried out a study of this concept for RSSB in 2015/6 - Project T1095. The Research in Brief summary is available (if you have an RSSB account) here:

https://www.rssb.co.uk/-/media/Proj...cts/2020/07/06/17/47/Research-brief-T1095.pdf

Here's the summary of findings:

Findings This research has helped to determine an environment where closer running could significantly enhance capacity on the GB rail network, without compromising current ‘safe separation’ principles. (This assumes certain rolling stock and operational improvements are in place.) It has identified the technology improvements that are needed to support the closer running concept: • ERTMS (European Rail Traffic Management System) Level 3 ‘moving block’ (a signalling block system where the blocks are defined in real time by computers as safe zones around each train). Automatic Train Operation (ATO) and Communications-Based Train Control (CBTC). • ‘Predictable braking’ to allow certainty that close running trains will all behave in the same way when braking. • ‘High-integrity switch technology’ currently under development (RSSB’s REPOINT research) is expected to help reduce the time for authorisation for movement across junctions of a subsequent train. • Significant improvements to traffic control and train flow information. An initial hazard and operations study (Hazop) carried out during this research concluded that, with these improvements in place, most identified closer running hazards could be mitigated. The only exception being for those resulting from a collision of the leading train with another object, such as an item of infrastructure, a derailed train, or a train travelling in the opposite direction. This was defined as a major hazard that, albeit low risk, cannot be fully mitigated. The implementation of the closer running concept beyond that afforded by the incremental improvements outlined above, would require a change in the established ‘safe separation’ approach within the rail industry. Some initial ideas contributing to the development of a road map to support the closer running concept, are summarised in Figure 1. They include the following steps: 1 ) Closer headways within CBTC/ ETCS system architecture 2 ) Convoy 3 ) Coupling and uncoupling on the move

In fact this work looked beyond the "slip coach" idea to see what effect it would have on system capacity if coupling on the move were possible as well. It concluded that it could radically transform the capacity of a railway. However this works mainly by allowing "slip" and "joining" portions to serve intermediate stops without delaying the through portion, and that really only works if all your intermediate stations have through tracks with platform loops - which (in my mind) puts it into the "pie in the sky" category. As @Bald Rick has said - it doesn't do anything for you at junctions anyway. As the summary says - what could, however, is the Loughborough University "REPOINT" idea which re-engineers the switch to be an inherently safe device with stable states in normal and reverse. I will try to dig something out on this later.