On Fri, 20 Aug 2010, MIG wrote:

On 20 Aug, 21:47, Eric wrote:
On 2010-08-20, David Cantrell wrote:

On Thu, Aug 19, 2010 at 09:48:11AM +0000, wrote:

Surely it has some sort of handbrake? How else could it be parked safely
for long periods when the air will have all leaked out?

Uh, aren't railway brakes held *off* by vacuum, as opposed to being held
*on* by air?

Vacuum brakes are held off by the vacuum, if the air gets in they go on.

Air brakes are held off by the air pressure, and if the air gets out
they go on.

How does this fit with the case of the trucks that ran away into St
Pancras after the brakes had gone on automatically but eventually
released?

(This was the time that the brakes went on as the driver started to
pull away, and he went to the "back" thinking someone had nicked his
back light and opened the pipe, without it occurring to him that the
coupling had broken, leaving a couple of trucks further back.)

Sounds like the main failure mode that air brakes have that vacuum brakes
don't: if you run out of air in your reservoir, then even in the absence
of pipe pressure, the brakes will release.

Again, i would have thought there would be a second-layer failsafe
mechanism that applies some other brake in the absence of reservoir
pressure (this could be as simple as a spring adding some air-independent
force to the brakes, requiring a surplus of pipe pressure over reservoir
pressure to release the brakes), but perhaps there isn't. Anyone know a
good book on railway brakes?

tom

--
Oh, and sometimes in order to survive you have to drink the irradiated
water from an old toilet. -- Jon, on Fallout