On 2008-10-13 22:43:59 +0100, Colin McKenzie said:

John B wrote:
On 11 Oct, 20:02, wrote:
But a new conventional 225km/h line to Manchester might be enough,
offering about 1h45, and the same argument could apply elsewhere.
Maybe only the Scottish run really needs more. Britain is smaller than
France or Spain, and thus the gains to be achieved from building LGVs
are proportionately less, particularly within England alone.

I'd be interested to see any studies on the cost per km of a new 225km/
h line versus the cost of a new LGV - and rather surprised if they
were significantly different.

The other issue no-one has mentioned is the cost (amount of energy)
used per mile of high speed rail travel compared to medium speed. With
efficient regenerative braking, most of the energy used is to overcome
friction, which rises with the square of speed - i.e. up to twice as
much energy is needed to go at 200 mph compared to 140 mph. This
matters because the main reason for preferring rail to air is reduced
CO2 emissions.

Admittedly it's easier to power trains than planes from non-fossil
fuel, but it's going to take a long time to get all our electricity
from renewable or nuclear sources.

I think 140 or 150 mph rail is fast enough for the UK. But that needs
to cover a lot more than a few principal routes, so that overall
journey time is not clobbered by 20 or 30 slow miles at each end.

The other factor in overall journey time is frequency - it's not much
use getting to Edinburgh in 2 hours if you have to wait another 2 hours
for the train to leave. That means we need increases in rail capacity
as well as line speed.

Colin McKenzie

I don't think it's quite that simple. It's not *friction* which rises
with the square of the speed, but the *air resistance*; friction
(wheel/rail interface losses, bearings and so on) rise proportionally
with speed.
The area under the speed-time curve corresponds to the energy used in
the journey for motion. So the total energy usage for a higher speed,
but shorter (in time) journey is not necessarily much greater than that
used in a lower speed, but longer in time, journey. Don't forget also
that 'hotel' power consumption (lighting, air conditioning, the coffee
machine and so on) is proportional to journey time.

And with a faster journey the train can do more journeys in a day, so
(for the same service) fewer trains are required.

I agree about the service frequency - one of the most effective ways to
reduce the apparent journey time of transport used by the public[1] is
by reducing the gap between successive trains, buses, planes or
whatever. This is important. after all, you never 'just miss' your car!

Anyway I'm not convinced of the argument that *new* high speed rail
routes are ecologically/environmentally/economically better than air
travel. After all, the only ground based infrastructure a plane needs
is a couple of miles of concrete at each end of the journey. Is it
sensible to try to build 200 miles and more of railway through some of
the most densely populated country in Europe? Unless a lot of money is
continually spent on railhead grinding and ensuring the trains' wheels
are round, high speed railways can be LOUD.

[1] On the basis that 'public transport' seems to refer only to trains
and buses :-)
--
Robert