In another thread about the former NYC 20th Century Limited observation car Hickory Creek, the following was posted: The '48 train never operated with steam....not even a second section. IIRC, the train was ordered with electro-pneumatic brakes, at least on some cars. Seems this was incompatible with Diesel power, let alone steam.
Actually, the HSC air brake equipment was capable of being controlled by steam, electric, or diesel-electric locomotives, as long as the locomotive's automatic brake valve was HSC capable. Most of the locomotives which were HSC capable used a modified 24-RL air brake schedule, but I believe there were also some with modified 8-EL (electrics and diesels) and 8-ET (steam) automatic brake valves which could operate HSC equipment. Only locomotives likely to pull HSC-equipped trains had the brake equipment.
The cars had air brakes that would operate conventionally when HSC was not in use. They used the brake pipe pressure to signal the control valve on each car to apply and release the train brakes. When HSC was in use, an electrical cable running from the engine through the train operated solenoids to signal the control valves. The cars then applied and released in unison, rather than applying and releasing first at the front of the train and working toward the rear. The perceived advantage was the ability to save seconds with each brake application and release due to not having to allow for the response time of conventional air braking. Personally, I don't believe there was any advantage in smooth train handling with HSC, if the Engineer was a good one, but I'll concede that a few seconds could be saved here and there. In case of an HSC failure, the brakes would also respond to brake pipe pressure and operate conventionally.
HSC (or,as it was sometimes called, E-P braking) was cut in or out on the locomotive and on each car. The cars were set up by the carmen at the initial terminal, but the Engineer could cut it in or out on the locomotive.
Santa Fe did not use HSC, but UP (and thus C&NW and SP) did. I believe the Burlington also used it, but am doubtful about WP and D&RGW on their respective portions of the California Zephyr. I have no idea which Eastern roads might have used it. It was only applied to streamlined trains.
It fell out of favor by the late 1950's, when dedicated streamliner consists became impractical.
