One of the criticisms of #HS2 is that because it's so fast, it uses up much more energy than normal trains - so Greens should push for lower speed railways. There is just enough truth in this to make it plausible, but it's not the whole story. A thread to explain why... /1
For sure, if you run a train twice as fast, it needs nearly four times as much energy to maintain speed. (check out Davis Equation if you want to know more. Aerodynamic resistance, that dominates at high speeds, goes up by the square of the speed). However .... /2
... other things to consider. Firstly, a better measure when comparing is how much energy is used to move *each passenger*; and if our interest is really CO2, it's how much greenhouse gas is generated to do that. /3
The magic measure for this is "effective grams of CO2 per passenger kilometre" - gCO2e/pkm. (The "effective" is to allow for the fact that it's not just CO2, and planes' emissions are more damaging) /4
This depends on the number of seats on the train - in the case of HS2 to Birmingham, 1100: quite a lot more than a normal intercity train - and the load factor: what proportion of those seats are occupied. HS2 anticipate loadings of 60% on their trains. /5
Having big trains carrying lots of people means that even if the train uses up more energy, per person it's lower. This is what we see with Eurostar compared to normal trains /6
Also important is how often do the trains stop. Stopping trains use up more energy than ones that just keep going, as it takes more to accelerate the train each time than to just go at constant speed. HS2 trains only stop infrequently, so don't waste so much energy this way. /7
And - because the trains are fast, they will be popular: they save people time. This has two benefits - 1: load factor goes up, reducing the gCO2/pkm; 2 - people use the train instead of their car: a CO2 saving. /8
Further, if the trains are fast enough, they will compete with domestic air routes. HS2 trains will link London and Glasgow nearly an hour faster than current trains so are competitive with the air journey. This is a massive carbon win as planes are so CO2-intensive. /9
And, as Greens, we would put in place policies that encourage people to go by train not car or plane. Higher load factor, more double-benefit. (As an aside, load factors on French TGVs are about 67% and rising, so HS2's estimates may be low: autorite-transports.fr/wp-content/upl…, 3.3) /10
Finally, train design is improving. New trains are already more than 10% more energy-efficient than the ones used to calculate the HS2 energy case (e.g. Siemens: railjournal.com/passenger/high…), through improved aerodynamics and lower weight. So the sq law doesn't really apply. /11
So, to sum up, HS2 is an energy- and CO2-saving opportunity. And we haven't even considered all the new local train-not-car journeys possible on the existing railway network, opened up because HS2 has replaced all the capacity-hogging express trains. Another thread... /end
