Diminishing returns of British high speed rail

The typical inter-city journey in Britain is about 100 miles. Now look at this table of speeds and journey times; remember that to achieve an average speed of 100 mph will involve a period of running at 120 mph or more.

• 30 mph, 3 h 20 m 
• 40 mph, 2 h 30 m 
• 50 mph, 2 h 
• 60 mph, 1 h 40 m 
• 70 mph, 1 h 26 m 
• 80 mph, 1 h 15 m 
• 90 mph, 1 h 7 m
• 100 mph 1 h 

The time savings for each successive 10 mph speed increment are 50, 30, 20, 14, 11, 8 and 6 minutes respectively. It is a situation of diminishing returns.

It gets worse than that, because there are break points. 50 mph is the maximum operating speed for light rail. After that, the regulations for heavy rail come in, which affects safety standards, signalling systems, vehicle design and track design, with engineering costs to match. At 100 mph, the EU’s Technical Standards for Interoperability apply, which are even more stringent; at over 125 mph, there is another step change, in design requirements, and for TGV speeds, yet another. All of these factors generate higher costs for construction and equipment, as well as additional wear-and-tear, which has implications for running costs.

There is more. The faster the running speed, the less the curvature, which restricts the choice of route. Then there is the physics. At speeds above 80 mph, air resistance is an increasingly important factor. Energy consumption is proportional to the square of the speed. This gives rise to a situation of accelerating costs, added to the diminishing returns referred to earlier.

Where stations are relatively close, there is little opportunity for running at top speed. For the stretch between Reading and Didcot, trains are approaching Pangbourne before they are running at 125 mph and are already braking soon after they have passed Cholsey.

All these factors give rise to high costs which means that sophisticated yield management systems must be used to fill each train; HS rail is not a walk on service. This means that passengers have to allow plenty of time in order to be sure of arriving at the terminus in time to catch the train for the journey they have paid for. Prudence would suggest between 30 minutes to an hour. This of course adds to effective journey time, resulting in the paradoxical situation that a slower, walk-on service can offer shorter transit than the high speed train where you arrive long in advance and then go and have a coffee while you wait for the train you have booked for.

HS rail is centre-to-centre, but most journey are not, and involve a local trip at one or both ends. It is much more cost-effective to cut times for local journeys, which carry a volume of traffic a couple of orders of magnitude greater than long distance routes.