As rcdrye touches on, the issue often wasn't supplying the heat, it was carrying enough water. The steam generator in say an F7 A unit would probably be enough to heat a train that a single F7 could haul, but an F7A only had room for a small water tank...I think something like 300 gallons, while in a B unit you could get a 1500 gallon tank. (In a B unit, the steam generator and water supply was located where the cab would have been on an A unit.) So a local train with say 4-5 cars that could have been easily pulled by one F unit might have needed an A-B consist to keep the cars heated if the trip was of any distance. Otherwise, they would have to make stops to resupply the water tank.
That's one reason GM developed the FP line, F units stretched to allow for a steam generator and large water supply.
Back to the original question...it's important to understand that the transition from steam to diesels wasn't 'all or nothing', and happened more quickly than anticipated in many cases.
When a railroad bought new streamlined cars c.1939, and E-units to pull them, those E-units were probably (except for some switchers) the only diesels the railroad had. If one of them went down, the railroad would use one of their passenger steam engines to fill in. It's easier to put a steam generator in an E-unit than to put HEP in a steam engine. Keep in mind, some railroads (like New York Central) were running trains of streamlined cars with regularly assigned steam engines into the mid-1950's.
Second, when a railroad bought new steam power, they assumed the engine's service life would be about 30 years. A railroad receiving new 4-8-4 engines in WW2 didn't anticipate that those engines might be scrapped before they reached 10 years of service. I think by the time the railroads realized steam would be virtually gone from mainline passenger service by the late 1950's, they had already invested in a fleet of steam heated passenger cars. Easier and cheaper to keep using what you have, rather than re-do all the engines and all the cars.
wjstixAs rcdrye touches on, the issue often wasn't supplying the heat, it was carrying enough water. The steam generator in say an F7 A unit would probably be enough to heat a train that a single F7 could haul, but an F7A only had room for a small water tank...I think something like 300 gallons, while in a B unit you could get a 1500 gallon tank. (In a B unit, the steam generator and water supply was located where the cab would have been on an A unit.) So a local train with say 4-5 cars that could have been easily pulled by one F unit might have needed an A-B consist to keep the cars heated if the trip was of any distance. Otherwise, they would have to make stops to resupply the water tank. That's one reason GM developed the FP line, F units stretched to allow for a steam generator and large water supply.
The B&O purchased a series of F3 A & B units with steam generators for use in passenger service, specifically the all new Vista-Dome equipped streamlined Columbian. Once placed in actual service it was found that the engines could not consistantly provide steam from the coach yard at Chicago to Grand Central Station for boarding and then on to Garrett, IN for a crew change and watering. The engines frequently arrived Garrett out of water. Subsequently the B&O used E units for all passenger runs out of Chicago. The F3's, with their passenger gearing were then assigned to the B&O's Chicago Division for use in fast freight service - and the passenger gearing defined fast.
Never too old to have a happy childhood!
A typical F3 or F7 with a boiler had a split 1200 gallon tank - 800 fuel and 400 water. Some units had a hatch tank in place of the dynamic brake hatch that would hold 600 gallons (these ones usually had the full 1200 gallon fuel tank). FP7s had space for two more tanks totalling 1150 gallons - an FP7 with a hatch tank could hold 1750 gallons without sacrificing fuel capacity. The smaller vapor 4616 (1600 lb/hr) S/Gs had a 200 gallon tank under the S/G itself. F7B units had 1200 gallon water tanks. The "Torpedo boat" GP7 and GP9 configuration moved the air reservoirs to the roof to allow for more fuel and water capacity.
wjstix Back to the original question...it's important to understand that the transition from steam to diesels wasn't 'all or nothing', and happened more quickly than anticipated in many cases. When a railroad bought new streamlined cars c.1939, and E-units to pull them, those E-units were probably (except for some switchers) the only diesels the railroad had. If one of them went down, the railroad would use one of their passenger steam engines to fill in. It's easier to put a steam generator in an E-unit than to put HEP in a steam engine. Keep in mind, some railroads (like New York Central) were running trains of streamlined cars with regularly assigned steam engines into the mid-1950's. Second, when a railroad bought new steam power, they assumed the engine's service life would be about 30 years. A railroad receiving new 4-8-4 engines in WW2 didn't anticipate that those engines might be scrapped before they reached 10 years of service. I think by the time the railroads realized steam would be virtually gone from mainline passenger service by the late 1950's, they had already invested in a fleet of steam heated passenger cars. Easier and cheaper to keep using what you have, rather than re-do all the engines and all the cars.
Interesting point on the expected life of steam locomotives. So what was the reasoning behind this? Was it thought that diesels would not be suitable for all situations, and a complement of steam power would be present on rosters for the forseable future? Or did the superior economics of the diesel exceed expectations, and the RR's realized they must retire all steam regardless of the relative young age of the locomotives?
A lot of railroads recieved steam power during WWII when they wanted diesels but the manufacturing capacity was needed for other things. They presumed that these locomotives would continue to live on in their fleets until their economic usefuless was over. This came more rapidly than expected however as wages (and thus steam costs) rose very rapidly in the 1950s, which made replacement with diesels attractive.
p.s. Another reason it was easier for Amtrak to do it is that they were one organization. Railroads passenger cars sometimes ended up on another railroad for part of it's journey, like say ATSF sleepers going from Chicago to New York City on a New York Central train. It wouldn't do any good for Santa Fe to convert to electrical heat if NYC was still using steam - or vice versa.
They had to be compatible; either they all change, or nobody changes.
Into the late 1960s the Pullman pool was also in play - a PRR sleeper might end up on just about anybody else's train for one or more trips. Conversion to HEP was also expensive. C&NW did it first for commuter trains, using former freight F7s and extra E8s, with Cummins engine/alternator packages, and then ordered the "400s" where any extra cars could be drafted from the commuter fleet. C&NW's first bilevels (StLCC 1955) were also among the first cars converted to HEP from steam to match the 1958 P-S order. The first were probably C&NW's diners and lounges which also got "hats" to match the bilevels' profile.
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