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Re: [Phys-L] train stopping distance

On 06/24/2016 01:09 PM, Bernard Cleyet wrote:

passengers ina train aren’t using seat belts.

An interesting point. A 0.5 g acceleration applied to people
standing around in a long, wide-open car could get ugly.

If I were the “engineer” of a passenger train, I’d make a quick
decision as to the conflicting result(s) of a one g stop and the
collision.

A 1 g stop is not possible. Physics says the best you can
do is about 0.5 g, limited by the coefficient of friction.

Typically a 0.5 g stop is not possible either. As detailed in
my previous note, typical equipment does not apply the brakes
hard enough to come anywhere near that. Something like 0.02 g
is more typical for a freight train. That seems kinda lame.

Braking force depends on details of the pneumatic mechanism.
The operating pressure has increased only slightly over
the years, while the mass of the trains has increased
enormously. You'd think they would increase the area of
the pneumatic pistons, but evidently they haven't.

More powerful brakes would require WSP (wheel slide protection)
which is the railroad equivalent of the ABS (antilock braking
system) found on cars. It exists, but isn't very widely
deployed on freight trains.

Also, as Bill N. pointed out, more powerful brakes might
do Bad Things to the tracks.

Note that brake system temperature is also an issue. I know
that after an episode of heavy braking, an airliner isn't
allowed to take off until the brakes have cooled off. The
brakes can absorb enough energy to stop the aircraft once,
but not twice in rapid succession.

Apparently emergency stopping distance is not something that
is given much emphasis by designers, operators, or regulators.
See e.g.
https://www.law.cornell.edu/cfr/text/49/232.103

The primary brake system of a train shall be capable of stopping the
train with a service application from its maximum operating speed
within the signal spacing existing on the track over which the train
is operating.

The train must have an emergency braking system, but its
stopping-distance performance doesn't appear to be covered
by this regulation.