Degrading practices, part 3

(… continued from Degrading practices, part 2, posted on 23 Mar 2015).

The timetabling of services on a railway has to be devised so that a “minimum safe headway” can be maintained at all times between any service running on a line and the service immediately following it on that line. The intention is to allow sufficient time, and thus distance, between trains for the trailing train to stop safely in the event that the leading train comes to a halt. The value of the headway is determined with reference to a number of factors, which include the maximum speed allowed on the relevant track section, the braking efficiency of all train types in use and the efficiency of the signalling system.

In order to allow the minimum safe headway to be maintained within the operational schedule, timetables have to take into account of the speed and stopping patterns of adjacent services, and this topic was the first subject addressed by Professor Andrew McNaughton, Technical Director of HS2 Ltd, when he gave his presentation on railway capacity to the HS2 Select Committee (see footnote 1). Speaking to exhibits P4557(3) to P4557(10), the professor told the Committee that “if all trains were the same and didn’t stop, then they can follow each other very closely and we get a lot of trains on to the network” (see footnote 2). This situation, he pointed out, is “the essence of what we’d be doing with High Speed 2” (see footnote 3).

Professor McNaughton said that “if all trains did the same thing” then you could run trains on the West Coast Main Line (WCML) “as little … as two minutes apart”, so that would be around thirty “train paths” in a peak hour on each of the two lines (see footnote 4). However, on a mixed-use railway, such as the WCML, gaps have to be left in this dense pattern of train paths to allow for services operating at different speeds and with different stopping patterns, in order that the minimum safe headway can be maintained. Professor McNaughton showed some simplified examples of how train path capacity gets “wasted”.

The table of services departing Euston that I introduced in part 2 can allow us to see how this theory is put into practice on the real WCML. This table covers the weekday peak hour from 17:00 to 17:59 hrs. On the fast line there are fifteen services departing Euston during that hour at the following minutes past the hour: 00, 03, 07, 10, 13(LM), 16(LM), 20, 23, 30, 33, 40, 43, 46(LM), 49(LM), and 57. These are Virgin Train services, except for the four indicated by “(LM)”, which are operated by London Midland.

The minimum spacing between two services leaving Euston is three minutes, rather than the two minutes cited by Professor McNaughton; presumably this allows for some operational tolerance (see footnote 5). It is possible to confirm that this is the practical minimum by examining the two services that depart at 00 and 03. The first train of this pair is a Virgin Pendolino that doesn’t stop until it reaches Stoke-on-Trent. The 03 departure is also a Virgin Pendolino but is slower than the leading train as it stops at Rugby and Coventry on its way to Birmingham New Street; so this allows for a minimum spacing.

Compare this with the service that leaves Euston at 40 minutes past the hour. This is another train that doesn’t stop until it is well up country; this time the first stop is Crewe. However, the immediately preceding train (33 minutes past the hour) stops at Rugby, which will cause the following train to catch up. To take account of this the gap between the scheduled times departing Euston has been increased to seven minutes.

A similar pattern can be seen on the slow line, although departure time spacings vary more widely here due to some extent, I assume, to the considerable variation in stopping patterns – the smallest space between two adjacent departures from Euston is four minutes and the largest is twelve minutes (see footnote 6). If the Southern service from South Croydon to Milton Keynes is included, there are eight services accommodated on the slow line in the peak hour.

Taking account of these operational requirements, it is fairly clear that peak-time train paths on the WCML out of London are pretty well used up, at least as far as the fast line goes. If the current service pattern is maintained, there are no obvious gaps to accommodate new paths on the existing tracks in peak periods; this means that, without other interventions, no new peak-time services can be introduced and no relief services can be run to ease crowded commuter trains. It is this dearth of new peak-time passenger train paths, and overcrowding on some commuter services, that lie behind the often-heard claims that the WCML is “full”. These claims are reinforced by a desire to make more off-peak train paths available for freight services.

However, there are capacity-increasing improvements that can be made that do not require new train paths, increasing the length of trains being the obvious example. That there is scope to do this appears to be confirmed by a leaflet published by London Midland, which shows that, of the eleven services that depart Euston between 17:00 and 17:59 hrs on a weekday, only four are in twelve-carriage configuration. The remaining seven have eight carriages only, and five of these are designated “standing room only”.

(To be continued …)

Footnotes:

  1. Professor McNaughton’s evidence to the Select Committee starts at 15:41 in the video and paragraph 153 of the transcript for the afternoon of Wednesday 11thFebruary 2015. His exhibits have been allocated identities P4557(1) to P4557(17).
  2. The section of Professor McNaughton’s presentation covering main line capacity is reported in paragraphs 157 to 165 of the transcript for the afternoon of Wednesday 11th February 2015.
  3. Whilst this claim is superficially correct, it takes no account of the possible difficulties of threading classic compatible HS2 trains into services on the WCML.
  4. A train path is the track access required to operate a single train service, requiring each section of track to be free of other services for a specific time to allow the free passage of that service.
  5. That the “planning headway” on the fast line out of Euston is three minutes is confirmed by figure 3.3 of the Network Rail document West Coast Main Line Route Utilisation Strategy (WCML RUS).
  6. Figure 3.3 of the WCML RUS confirms that the planning headway on the slow line out of Euston is four minutes. Beleben attributes this constraint on the line’s capacity to the signalling system, so this would be an obvious area where a moderate investment could increase the throughput. Even with the present technology, there should, I think, be some scope for increasing the number of peak-hour services above the eight currently running.

PS: Whilst I have tried very hard to get my facts, and interpretations that follow, right, I am very conscious that I am not a railway buff, but that some of my readers are. If I get anything in this current series wrong, please let me know.

Important Note: The account of the proceedings of the HS2 Select Committee that is given in this blog is based upon an uncorrected transcript of evidence, which is not yet an approved formal record of the proceedings of the HS2 Select Committee. Neither witnesses nor Members have had the opportunity to correct the record, and it may therefore be subject to changes being made in the light of any such corrections being requested.

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