Gil,
The UP "Pacific" 10-6 sleepers were equipped with electro-mechanical A/C as built and did not carry any clearance-limiting features, so far as I recall, and as such, were widely employed systemwide in the early Amtrak days. I cannot imagine any barriers to interline use of these cars in the pre-Amtrak days.
In my earlier post I described an exacting clearance process that was specific to either a number of identical cars by "lot", or by individual cars on a case-by-case basis. Placement of various underfloor appliances varied greatly, which revealed some exceptions that could pass clearance. Although I recall the AT&SF conventional fleet was predominantly SE equipped, I do believe they did have some EM equipped cars.
In the early-to-mid 1950's, when standards of service were still high, it would have been possible to include a steam-ejector equipped transcon sleeper - clearance details permitting - as the custom was then to provide continuous steam on the premier trains. This was the only way to furnish hot water in the cars. Steam was protected "12 Months" of the year.
By the time of PC, however, steam was protected for heating on a seasonal basis only, between the months of October and May, IIRC. Without steam through the late Spring/Summer months, passengers simply were not provided hot water for washing. By no small coincidence, there was practically no maintenance performed on hot water systems in those days.
In the early days, around early Summer 1973, IIRC, when Amtrak extended the Southern Crescent to Boston, the through portion of the train was Southern equipment, mostly steam ejector equipped in a manner that cleared NYP. I remember an arrival at South Station (possibly the first such run-through) one sultry evening with no steam and the passengers detraining in the manner of a swarm of angry hornets!
Nobody at HQ thought of protecting steam, and to the mechanical forces at Boston (mostly dedicated and highly skilled with NYNH&H seniority) the use of steam to produce AC was an entirely alien concept. Overnight, they heroically set up the returning power with four working boilers, observing a strong ejector vacuum developing on each of these cars. In short order, Southern replaced the SE with EM cars in through service.
As I know you can imagine, SE equipment (though reliable by its design) simply did not mesh in the network operation that Amtrak was becoming.
With best regards,
Paul.