Here’s the first of what may well become a new feature. Guest writing from friends and readers of this blog. The expression “rail expert” is terribly overused. Most of the time it’s used to refer to journalists like myself who may know about the railways. We wouldn’t dream of using it of ourselves, because the more we know, the more we realise how little we know. However, I think that if anyone can be classed as an expert in his subject it’s William, so I commend this blog to the house! William sent this to the Lords Economic Committee in response to Lord Tony Berkeley’s recent evidence to said committee.

Yesterday, you took evidence from Lord Berkeley to the effect that the business case for HS2 should be based only on 14 trains per hour (tph), as the nominal capacity of 18 tph on the HS2 trunk route is impracticable. He is wrong.

First, consider the theory, because if the theory doesn’t work there is no further argument. Taking into account realistic braking rates, the minimum time separation of trains at 360 kph can be calculated to be less than 120 seconds. For this calculation, please refer to my article in the Institution of Railway Signal Engineers journal ‘IRSE News’ for May 2019. More detailed modelling reflecting local features and constraints shows that this can increase to around 130 seconds. But it is clear that a flow of well above 18 trains per hour is possible on this basis.

Counter intuitively, though, the plain line headway at full speed in open air is not the binding constraint on capacity of the route. The highest headways arise in slow speed areas around stations particularly where approached through tunnels. The first implication of this is that the top speed of trains is irrelevant to capacity in practice, as raising it further would not create a new binding constraint, nor would reducing it ease a binding constraint.

The second implication is that it is these slow speed areas that need to be considered when assessing feasibility of the proposed capacity. HS2 has a technical standard that headways should be generally 120 seconds but not above 150 seconds. Modelling shows that even in the slow speed areas, 150 seconds technical headway is achievable. This is significant, as the International Union of Railways (UIC) guideline on practical exploitation of theoretical capacity is 75% for a dedicated high speed railway at peak periods. 150 seconds multiplied by 18 trains per hour gives precisely that figure. So using real data for the worst headways and the guidance of an international body, 18 trains per hour is feasible.

At this point, please note that, contrary to the evidence of Lord Berkeley, the HS2 business case is based on only 17 tph, with one further path pencilled in for possible future use. So the business case is even more robust in this respect.

That’s the theory. The issue then becomes whether the planned HS2 service can be operated reliably in practice, particularly in view of the risk of late handovers from the conventional railway. Frankly we can’t know until we try it, and that is no more or less the case for HS2 than for any other railway, but in accepting that one must also recognise that there is always management action possible to influence reliability, and that identifying a risk effectively means identifying where management action needs to be focused. One must also consider how robust HS2 can be expected to be in the face of such imposed delays, and the answer is – more robust than a classic railway.

The two big risks to service performance in the face of imposed delays are a) fast trains ending up following stopping trains once trains are running out of order, and b) conflicting moves at termini as the planned pattern of arriving and departing trains degrades. In the case of HS2, a) can be dismissed instantly, as there are no fast and stopping trains on the trunk route – all trains are running at the same speed, and all stopping only at the one station at Old Oak Common. If a later running train has to squeeze in to the flow, the consequent delay will decay over the next few trains. Moreover, the train that finds itself following a gap left by a late train can speed up in open air into that gap, thus allowing the delay to be eroded both before and after the late train. Then, b) is mitigated by the Euston track layout, which incorporates features that preserve robustness. Prime amongst these is a grade-separation in the station throat that makes the layout effectively two half stations handling half the service each, and so removes most of the scope for conflicts between out of course trains. Moreover, within each half-station, a platforming pattern that steps across from arrival side to departure side means that a later-running arrival will not normally create a conflict, as it will be in parallel with any later departure. For trains presenting seriously late, the option of turning back from Old Oak Common exists and has no parallel on the existing railway.

Lord Berkeley’s suggestion that 14 tph should be the maximum is based on two things:

1. The UIC calculation outlined above, except that he has wrongly and inexplicably applied the 75% factor twice, first to reduce the theoretical 24 tph at 150 seconds separation to 18 tph, then again to reduce that 18 tph to 13.5 which he approximates to 14 tph;
2. Evidence given many years ago, primarily to the Commons Transport Select Committee, by representatives of the French railways that they consider 14 tph or thereabouts to be their practicable maximum. Astonishingly, no-one ever asked them why! What feature of their railway sets this limit? From my argument above, I think it is unlikely to be the technical headway at full speed, unless a considerably less-than-state-of-the-current-art signalling system was in question. Where HS2 is constrained to 18 tph, as above in the slow speed areas, it is in effect like any other railway, many of which operate 18 tph now

I contend that the view expressed to you by Lord Berkeley yesterday is unfounded both theoretically and in practice. Please do not hesitate to contact me to discuss these issues further if this is helpful to you in reaching valid conclusions.

Regards,

William