From: johnl@iecc.com

>>ObRail: for a truly bad idea, imagine a railcar designed by someone who's
>>previous effort has been on the composite body B787
>
>If composites come down in price, would they be a good idea for rail
>cars?

Interesting question, but I'd be surprised because the requirements
for planes and trains are so different.

For a plane, you need to minimize the weight since the engines are
keeping the plane in the sky, you need to maximize strength, but you
don't care much about rigidity.  It's OK if stuff deforms so long as
it recovers.  (There's an impressive youtube video of a B777 wing
strength test where it deforms to about 75 degrees above horizontal
before breaking.)

Safety concerns are primarily about keeping the plane flyable despite
multiple failures so the pilots can land it safely, and then evacuate
fast before leaking fuel catches fire.  It's vanishingly rare for a
plane to hit something, and not a coincidence that the most deadly
plane accident ever happened when two 747's collided on the ground in
the fog at an airport in Spain.

Trains are different.  The train's wheels hold it up, and since steel
on steel has such low friction, the main energy uses are accelerating
and decelerating, and the latter can often been recovered by dynamic
braking.  Trains bounce around like crazy, particularly if the track
is at all rough, and up to a point more weight makes a train ride more
smoothly.  There's certainly weight/performance tradeoffs, with TGVs'
being a good example, high power to weight so they can climb steeper
grades than conventional trains which allowed them more latitude in
picking new routes for them.  But it's still nothing like a plane.

Rigidity matters, since an insufficiently rigid train will derail or
sideswipe something.  Trains also tend to run in much more hostile
physical environments, such as the snow that Adam K.  mentioned, and
salt from deicing and nearby roads.

Train accidents generally involve derailing and/or hitting something
like another train, so the safety features require the cars to be
rigid enough to survive the accident without crushing the passengers,
and for something to absorb the train's energy slowly enough that
there isn't a sudden jolt that sends people flying through the
windows.  Collapsible engine noses are supposed to help.

So anyway, composites certainly don't rust, but I don't think they're
particularly rigid, and the light weight doesn't matter.  There are
buses with plastic trim panels that one can remove and replace if
they're damaged, so I can see uses like that, but it seems unlikely
for anything important.

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