Steam Excursions... and brakes?

[Overmod]

American locomotives have not used steam driver brakes for many years. (Nor have they used steam for the power reverse, even though the British had notable success with steam (the Hadfield) since their brakes largely involved vacuum exhausting and not pressure air.

The general device used on any locomotive of particular size was the Westinghouse cross-compound air compressor, which was a two-stage device using steam pressure in relatively long expansion to make air pressure.  These used only reliable enclosed valves and relatively cheap piston seals to work, and were highly efficient at converting steam mass flow to air at high pressure -- more than any steam-powered rotary compressor would likely be.  Two of these were used where additional volume -- for example pumping up the reservoirs and main brake line on long cuts of cars or standing trains -- was desirable.  (In general the reason for two compressors was not redundancy, and was not 'sequential compression' to typical main-reservoir pressure as each compressor did this via compound expansion within itself.)

The considerations where to put these are interesting, as they could be physically located almost anywhere on a locomotive and did not have to be near each other.  They therefore became a useful way to balance locomotives both longitudinally and 'side to side', with the further consideration that mounting them on increasingly high-pressure boilers could be a pain, but expensive and heavy bracketing would be needed to support them from the frame on either side; running them exposed to weather and various contaminants and impact damage could be bad, and the air they compressed had to be cooled and the condensed water in it separated and discharged.  One ideal place was near the pilot beam, where a frame to hold them could be provided, shielding easily provided but maintenance access and inspection still easy, and cool air for a 'radiator' lighter and more effective than a few runs of brake piping up and down a running board could be was available.

Once you require a source of air pressure to run the automatic brake, it becomes sensible to run the independent on the locomotive the same way -- much fewer issues with corrosion, condensation traps or freezing, and you can arrange to coordinate train and locomotive brakes in automatic-brake applications easily (with 'bailing off' then being possible, and often desirable, in many circumstances.)  What was not immediately recognized was that reliable main-reservoir air could also power other things on locomotives effectively, including power reverse and, later, front-end throttles without complicated and balky mechanical linkage, booster control, and air horns to mitigate the loss of water mass and heat involved in whistle blowing.

It also bears looking into why some other systems on the locomotive were not designed to use reservoir air to operate.  Most practical stokers used steam motors (as high power and flow were required, which would chill things down if compressed air were used) and steam rather than air jets in distribution for similar reasons.  For fun you can study the use of compressed air vs. steam for antismoke overfire jets or 'guns' as applied to late steam power -  either method had its perceived advantages and disadvantages.

(Incidentally a request to Al Krug on the current location of his 'rrfacts' discussion of air brakes has recently borne fruit: he has uploaded it to the server he uses for 'Krugtales' and that is the current 'best place' to find it.  This is well worth bookmarking and perhaps putting in a 'sticky' reference on useful sources of information...)

http://krugtales.50megs.com/rrpictale/rrfacts/rrfacts.htm 

 

[tree68]

There are definitely air compressors on steam engines.  Look for a couple of vertical cylinders (usually) on the side or the front of the loco.  They will be one above the other, and a close look between them will often reveal the moving piston rod.

If a steam engine is at rest, you'll occasionally hear a "chuff" - that's the air pump.

Some railroads liked to mount the compressors on their big locos on the front of the firebox - made for a pretty fearsome appearance.  Others put the compressor assembly(s) behind covers on the front of the unit.  Most often, though, you'll find it on the fireman's side of the boiler.

Aside from that, the brakes (and controls) are/were essentially the same.

[steve-in-kville]

Do the steam loco's used for excursions (like RBMN) have a means to produce compressed air to stop their passenger cars? I understand that they use steam to stop the loco, but what about everything behind it?