As of October 1976, I don't recall a first class dome or diner, If I am recalling correctly, I recall that I walked the whole train one night during a trip back to Chicago, and there were just sleepers. Diner and lounge were the old pre Amtrak-bi level El Cap cars.

These were the leanest times for the still "new" Amtrak. Amtrak was under-capitalized and intended by some to fail. Amtrak was stuck with a quickly aging equipment pool from the former passenger operators. Suddenly, Amtrak has a surge in post oil-shock ridership increases and widespread public support of more rail service. Diners and lounges were old and in short supply for all of the long distance trains, so a double set of diners or loungers on the Southwest Limited was not a wise allocation of resources, compared to providing more lounge capacity for food service with the new demand to be served on things like newly revived LOSSAN corridor.

HEP was still largely a theory back then. So trains soon to be discontinued like the aptly named National "Limited", operated with "heritage" equipment, into the winter of 1977, complete with frozen steam lines and resulting in no heat in some cars. The National Limited was also offering service "limited" to hundreds of miles of 25 mph slow orders on the former mainline from Pittsburg to Saint Louis and Kansas City, due to the financial collapse of the eastern roads, and years of deferred maintenance. Crossing central Ohio or Indiana can seem like crossing Texas, when you're traveling at the amazing speed of 25 mph......

The reduction in these amenities was one to the issues with The use of AT&SF's Super Chief name, which would have happened before that name change.