I lived in the Twin Cities in 1976 and traveled on the Empire Builder quite a bit. I traveled on the Broadway LTD from Philly to Chicago in March, 1973 Also traveled from Washington, DC to Minneapolis in May, 1977 on the Broadway LTD and Empire Builder. In the early 70s, Amtrak ran 2 trains each way between Chicago and Minneapolis (before the St. Paul Midway Station). The North Coast Hiawatha continued west of the Twin Cities via the former NP route 3 times per week, but was daily in some of the summers. The Empire Builder carried pretty much the same equipment that it had under BN/GN heritage. There were usually 3 sleepers to Seattle, a sleeper to Portland, a slumbercoach to Seattle, a diner to Seattle, a Great Dome lounge to Seattle, 5-6 coaches including at leat 2 domes. The Portland section usually had a diner/lounge as it had in pre-Amtrak days. Often times former CZ cars were used on the Empire Builder, but more often the North Coast Hiawatha. The North Coast Hi often carried a CZ dome observation lounge between Chicago and Seattle. The days when the Hiawatha only went to the Twin Cities it carried a Burlington Dome Parlor Lounge origally built for the Denver Zephyr, KC Zephyr or Twin Cities Zephyr.
The Broadway LTD carried 2-3 sleepers from NYC to Chicago, a slumbercoach from NYC to Chicago, a sleeper or two from Washington to Chicago that was combined at Harrisburg, a double unit diner from NYC to Chicago, a snack lounge from Washington that I think also was combined to Chicago, 6-8 coaches of which 2 originated in Washington, and a bedroom observation car that originally ran on B&O's Capitol Ltd.
Both the Broadway LTD and the Empire Builder added cars during the summer season. There many excess cars from the Western Railroads that were in good condition (better than some of the cars now) that were used on both eastern and western trains. I remember all the trains being quite busy in the 1970s because prior to Amtrak there were a lot more trains running so the few that were left did well. This all seemed to change in the late 70s when the first cost cutting measures were implemented.