[Animats]

"These are the pressurized air brakes on trains, in which air pressure holds the brake shoes open against spring pressure." Air brakes don't really work that way.[1] There's an air tank on each car to provide the pressure to apply the brakes if the brake line loses pressure.

Fail-safe design comes from railroad signaling. It is a principle of classic railroad signaling that any broken wire or relay that fails to pull in must result in an indication not less safe than the correct one. "Vital" Relays in classic signaling systems fall open by gravity, and use silver-to-silver contacts so as to avoid welding together on overloads. (Lightning strikes on rails and on signal lines are considered a normal part of railroad operation.)

[1] https://en.wikipedia.org/wiki/Railway_air_brake#Straight_air...

[dredmorbius]

From your linked source:

"Under the Westinghouse system, therefore, brakes are applied by reducing train line pressure and released by increasing train line pressure. The Westinghouse system is thus fail safe—any failure in the train line, including a separation ("break-in-two") of the train, will cause a loss of train line pressure, causing the brakes to be applied and bringing the train to a stop, thus preventing a runaway train."

Without air pressure -- from line or cannister, the brakes fail in the activated mode.

I'm trying to find a source, but my understanding is that red/green for lit signals as "stop/go" came about after an earlier mode, in which a steady white light meant "go" proved problematic: the red disks fronting stop lamps could fall out (or perhaps be broken), leaving ambiguity as to what "white" meant.

Switching to red and green lamps meant that the failed-disk mode now clearly indicated a signalling problem, where the signal could not be trusted.

[Animats]

No, train brakes need pressure from the car tank to be applied. This is what the famous "triple valve" is for. High train line pressure releases the brakes and charges up the car tank. Low pressure applies the brakes. This has the annoying property that you can't leave a train parked on a grade for too long without applying the manual brakes on the cars. US freight air brakes were standardized in 1893, and haven't changed much since.[1]

Semitrailer parking brakes really are spring-loaded and released by air pressure.

[1] http://www.railway-technical.com/air-brakes.shtml

[dredmorbius]

OK, Today I Learned.

This diagram in particular (from your URL) shows that though there is a spring in the brake-shoe application mechanism, its action is to release the brake.

I hadn't know this (and had never found a good diagram of railroad brake design). This isn't what my understanding had been.

NB: this isn't my area of expertise, and my understanding had been the incorrect idea that spring-pressure held brake shoes in place.

Which makes me wonder why this design was chosen over a spring-driven shoe.

Thanks for sharing that. And brickbats to the hive-minders who've (at this point) downvoted your earlier comment in this thread.

[Animats]

"Which makes me wonder why this design was chosen over a spring-driven shoe."

The real answer is that the Westinghouse air brake system won the 1887 Burlington brake trials. Other entries included vacuum brakes, buffer brakes (bumping into the car ahead applied the brakes), a competing air brake system, and electropneumatic brakes (by Herman Hollerith, the punch-card guy). Nobody entered a spring-loaded system.

[dredmorbius]

And you've got standardisation, across an entire rail ecosystm (that's rail, not Rails), in which locomotives, rolling stock, couplings, etc., etc., etc., all need to work together.

An advantage of standardisation is you get, well, standardisation. Such as US President Herbert Hoover implemented by setting up the National Institute for Standards and Technology (NIST), which specified standards for screws and nuts and bolts. I'm not sure if Bendix transmissions were included, but come WWII, it was possible for the US War Department to order something like five million Jeep transmissions from several dozen suppliers, any of which could (at least in theory) be interchanged or have parts swapped between them.

The disadvantage is that you may find yourself very effectively stuck at a local optimum that's far from a global optimum, with murderous path dependencies.

I've been grousing over a set of TV propaganda videos created by the Mont Pelerin Society / Cato Institution through Johan Norberg and his "Free to Choose Media" production company (at least the propaganda slant is fairly obvious). The 2nd installement of his series on Adam Smith spends much of its time aboard a supersized cargo carrier, waxing rhapsodic about the wonders of the market in coming up with such a marvelously efficient system.

Except that it took the US Navy to standardise container sizes. After some 20 years of dickering over container sizes, materiel transport needs of the Vietnam War finally forced standarisation.

(Another US regulatory body, the Interstate Commerce Commission, meanwhile, had been happily impeding progress thanks to its regulatory capture by the railroad industry, and I won't even begin to mention the Texas Railroad Commission, which has little to do with railroads and was exceptionally significant well beyond Texas, at least for a time).

[mcpherrinm]

The Lac-Megantic crude oil train derailment/fire disaster is rather horrific example of that "annoying" property, where an insufficient number of manual brakes were applied, and an engine fire caused the engine proving air pressure to be shut down.

https://en.wikipedia.org/wiki/Lac-M%C3%A9gantic_rail_disaste...

[dredmorbius]

I was aware of brake failure as a factor in the Lac-Megantic disaster, but not that this was the specific cause.

[dredmorbius]

NB: it's really annoying to see HNers downvoting factually accurate and well-intentioned comments.

Particularly when they're correcting errors or omissions in other comments. Such as those in mine above to which Animats is replying.