You want to go how fast?

The effects of speed, part 1

When the plans for HS2 were being concocted in some dark corner of Marsham Street (or possibly Victoria Street) the word appears to have gone out that HS2 should be “ultra-fast” and future proofed.

The result is that the trackway has been designed to support a maximum running speed of 400 kph (250 mph) over the greater part of its length (from just north of Little Missenden to the vicinity of the National Exhibition Centre, near Birmingham). As I said in my blog of 23 Apr, this decision impacts on the environmental, social and cultural consequences of HS2; in this and the next four blogs I will explain why I think this.

The maximum running speed specification influences the track geometry and to support 400 kph HS2 Ltd has specified a minimum curve radius of 8,200 metres. This dictates that the route must generally follow a straight-line path, with only very shallow curves. This inherent inflexibility makes it problematic to divert the route around sensitive areas, which may be natural environments that should be preserved, sites of cultural heritage or just places where people live.

In my village, Cubbington, we experienced the impact of this inflexibility when HS2 Ltd “realigned” the route last September; this was done to take the line of the route away from the nearby village of Stoneleigh, which is about 4.5 km north of Cubbington along the line of the track. The aim of the realignment appeared to be to move the track about 500 metres further away from Stoneleigh, but retain the original alignment as soon as possible on the sections of track north and south of Stoneleigh.

However because of the constraints imposed by the minimum curve radius, the displacement from the original alignment is still about 150 metres when the track reaches Cubbington, bringing HS2 closer to our village. There is a similar effect to the north of Stoneleigh, where Kenilworth suffers. This seems to be a feature of HS2 design; if the design is changed to improve the situation for one settlement, then another is almost bound to suffer a worse situation.

Incidentally, the reason why a minimum curve radius is specified is not, as you might think, because the train is in danger of becoming derailed if the curves are too tight. The problem, I have been told, is rail wear. The higher the normal speed at which a curve is taken, the greater will be the cumulative wear. Increasing the minimum curve radius specified will reduce this wear and, thus, the maintenance overhead.

The maximum running speed specification also has an influence on the ability to vary the vertical alignment of the track to reduce the visual and noise impacts. This is because if variations in the track grade are too abrupt the passengers will influence an uncomfortable “switchback” effect and the resulting level of passenger discomfort will increase at higher speed. This leads to the specification of a minimum radius for the “vertical crest curve”, which HS2 Ltd has set to 56,000 metres.

In the next blog I will look at the effects of the maximum running speed specification on fuel consumption.


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