Class 717 - New trains for Northern City Line & GN

Nov 26, 2018 18:54:35 GMT goldenarrow said:

According to a Network Rail engineer, most likely. The entirety of the Thameslink core is signalled for 30mph and it is likely that Siemens will provide the signalling equipment for European Train Control System Level 2, other than that I don't think the state of the track could warrant higher speeds.

The introduction of Automatic Train Operation (ATO) is always an opportunity to review speed limits. ATO can be much fussier than manual driving would typically be and will be able to reaccelerate to take advantage of a short section of higher permitted speed, where a driver would typically coast. Maximum ATO speeds can be more finely tuned and change several times within a station to station link for short sections. Where the track geometry would allow a speed limit of say 48 km/h, this can be taken advantage of, where the signed speed might have to be 45 km/h. While these may just be a few seconds on a particular link, repeated train after train they may well deliver a significant benefit and have definitely been taken account of when making the case for the upgrades that I have been involved in. Whether there is a similar opportunity through the Thameslink core, when it converts to ATO I do not know, but I would hope that this was considered by the signalling designers. It should be more widely applicable to National Rail services as ETCS is rolled out more widely.

Across whole lines, such savings are doubly valuable, not just benefiting passengers, but often reducing the number of trains (and hence drivers) and hence cost required to deliver a given service level. It should be borne in mind that the lower the starting speed, the more time is saved by each increment of 5 km/h. You will save much more time by lifting a 25 km/h speed to 30 km/h (24 seconds per km), than you will raising 80 km/h to 85 km/h (2.64 seconds per km). An uplift of the Moorgate branch speed from 30 mph to 40 mph applied uniformly (I am not suggesting this is necessarily feasible) would save 18.6 seconds per km. If this time saving were achieved across the four links between Drayton Park and Moorgate (and bear in mind that I do not know the actual distances), the saving would be about a minute and fifteen seconds in each direction.

Nov 27, 2018 18:05:23 GMT norbitonflyer said:

CULG gives distances (converted from the imperial mileages actually used) between stations in the tunnel section as 680, 830, 1870 and 820 metres respectively. (Total 4.2km).

However, the time saving of 18.6sec/km could only be achieved if the train could run at the maximum line speed throughout. But it would need to brake for each stop and accelerate again afterwards. Assuming both are at a rate of 0.1g (1 metre/sec/sec) (and no Moorgate control at the terminus!), I calculate the time saving between Drayton Park and Moorgate by raising the speed limit would be less than a minute.

Sorry, my first post was a very rough simplification and I had assumed that the links were much longer and would have a clear kilometre each at speed – hence I didn’t bother with the acceleration and braking, although my having this impression perhaps underlines how slow the line is!

I have put the above distances (thanks very much for these) through my run time calculator with the following assumptions:
1. That the 10 mph speed limit at Moorgate applies for 250 m of this link in either direction (the straight exit route from one platform could presumably have a higher speed).
2. That Class 717 performance is roughly on a par with typical modern metro rolling stock with which I am familiar, and has an average acceleration rate of 0.77 m/s/s and can use an ‘in tunnel’ non-emergency brake rate of -1 m/s/s in ATO. The brake rate would be set quite a lot lower than this on open sections of track and would depend on the adhesion characteristics likely to be encountered.
3. That the whole section is level track (clearly this won’t be true, but shouldn’t hugely change the difference between the speeds unless the gradients are severe).
4. That the performance of the train is indifferent to load (again clearly not true, but this will make a few seconds difference to the overall times, but won’t change the saving much).

Assuming the 680 metre link is at the Moorgate end, I get a saving of 54 seconds from Moorgate to Drayton Park and 51.5 seconds from Drayton Park, although there is little difference between Old Street and Moorgate. Even if the actual realised saving with ATO is only say 90 seconds on a round trip, this would be a very valuable saving of passenger journey time. ATO will tend to allow more aggressive acceleration and braking of course and that effect is not accounted for in the calculation…