Given the walking limitation, in my limited airport experiences, I'd go with MKE, PDX, or DEN. BWI would work, too. However, given the walking limitation, all airports will require wheelchair/red cap assistance.
MKE works because it's a short walk from the platform to the free airport shuttle van which runs about every 20-30 mins. But the Empire Builder does not stop at Milwaukee Airport (MKA), so you'd have to take a Hiawatha to MKE or CHI to catch a western train.
At PDX, the light rail station is in the airport, but as mentioned by another poster above, would require a change to the green line and would be your limit af walking to the PDX statiom. The transfer to the yellow line is a 1 block walk...I did it a couple weeks ago.
I haven't been to the new DEN airport yet, so I don't know how convenient that would be.
BWI requires a shuttle van to the BWI station on the NEC. Or, take the light rail line and transfer to the BAL station line at Camden Yard.
I specifically do NOT include either Chicago airport as the distance from either the L (from MDW) or Dearborn subway (ORD) is a 5-6 BLOCK walk to Union Station!
PHL is a contender too, but as stated, you've had your fill of #29/30 and #48/49. Have you tried #50/51? That's the best scenery to CHI in my opinion. The meals are almost the same as on #48/49 'Diner Light' until 5/25.