So I figured out how to find the average number of seats on each Amtrak route thanks to the passenger-miles per train-miles that the FRA provides and the occupancy figures that you can derive from Amtrak's monthly performance reports. This here is for FY13. Should be handy in advocating for lengthening particular trains (such as the Carolinian, which could probably turn a profit with another car).
Code:
Train Average seats per train
Acela 312
Keystone 335
Northeast Regional 442
—Richmond/Newport News 437
—Lynchburg 506
—Norfolk 510
—New Haven/Springfield 242
Capitol Corridor 314
Carolinian 331
Cascades 261
Downeaster 290
Adirondack 270
Empire Service 388
Ethan Allen Express 387
Maple Leaf 203
Heartland Flyer 224
Hiawatha 406
Hoosier State 153
Illinois Zephyr 266
Illini/Saluki 332
Lincoln Service 311
Blue Water 361
Pere Marquette 221
Wolverine 396
Kansas City-St. Louis 178
Pacific Surfliner 474
Pennsylvanian 303
Piedmont 159
San Joaquins 315
Vermonter 298
Auto Train 512
California Zephyr 294
Capitol Limited 289
Cardinal 225
City of New Orleans 262
Coast Starlight 356
Crescent 303
Empire Builder 352
Lake Shore Limited 384
Palmetto 319
Silver Meteor 357
Silver Star 307
Southwest Chief 290
Sunset Limited 264
Texas Eagle 259