Amtrak in the Heartland by Craig Sanders details the changes in Amtrak's various routes/trains which serve and have served the "Heartland," which he essentially defines as any trains operating to, through, or from the Midwest (basically all Chicago trains plus a few corridors that are appendages of Chicago trains). In the book, some ridership figures are listed, but unfortunately it does not list every train every year. With that in mind, here is what it has (with 2011 figures for comparison):
Cardinal
1975 - 111,965
1976 - 129,294
2011 - 110,923
Empire Builder
1973 - 363,100
1974 - 385,300
1975 - 324,639
1976 - 311,576
1977 - 297,180
2011 - 469,167
Floridian
1974 - 157,198
1975 - 139,670
1976 - 153,054
1977 - 146,500
Inter-American/Eagle/Texas Eagle
1974 - 48,851
1975 - 40,476
1977 - 144,477
2011 - 299,508
National Limited
1975 - 163,422
1976 - 179,213
1977 - 191,692
North Coast Hiawatha
1973 - 227,200
1974 - 280,800
1975 - 208,809
1976 - 191,615
1977 - 205,642
Panama Limited/City of New Orleans
1974 - 195,899
1975 - 174,014
2011 - 233,318
Super Chief/Southwest Limited/Southwest Chief
1974 - 327,000, with a note that most years in the 1970s hovered around 280,000
2011 - 354,912
Texas Chief/Lone Star
1973 - 257,800
1974 - 277,760
1975 - 255,881
1976 - 274,448
Hopefully this it at least somewhat helpful.