On Jul 26, 5:58*pm, "Peter Masson" wrote:
"Andy" wrote



The current used by the system is NOT the amount of waste heat
generated by the train. You keep mentioning the power consumption, but
this has nothing directly to do with the amount of waste heat
generated by the trains (for a given design it will be related of
course, but not between designs). I have never said that the trains
will be consuming less power, just trying to point out that the power
will be used much more efficiently in a modern electrical system than
it was in a 1960s design.

The electrical energy introduced into the system does end up as heat. It
might on the way be used to accelerate the train, converting electrical
energy into kinetic energy, but this will all end up as heat, whether
through friction or braking. If regenerative braking puts electrical energy
back into the system, this reduces the electrical energy introduced into the
system.

Essentially all the electrical energy ends up as heat, and has to be removed
from the system through ventilation. You can't break the First Law (of
Thermodynamics).


I don't disagree. But as you say, it is the whole system, not just one
train. A much smaller percentage of the electrical energy that the
train itself uses, with regeneration, is going to end up as heat than
with rheostatic braking. The heat that is produced will be more spread
out as well. With rheostatic braking, the resistor bank is a
concentrated 'hot spot'. Much of the remaining kinetic energy will go
into movements of the air and so heat will get deposited in other
parts of the stations than the platforms (as well as going into
ventilation shafts). Just think of the breeze that can be felt for
quite a way (including up the escalator shafts) when a train arrives
into a platform. Heat energy is the internal vibrations of molecules
(both in the air and surroundings in a station), whilst the breeze /
bulk motion of the air is still kinetic energy.

In addition, some kinetic energy will be converted to potential energy
when stopping at stations with a hump (uphill into platform, downhill
out of platform). The potential energy works better for the lines that
go above ground, of course, as the kinetic energy underground becomes
potential energy when the train surfaces.