Some airline's boxes are as large as Amtrak's boxes, allowing you to just loosen the stem and remove the pedals and roll your bike into the box.

My bike in the background is just the way it came out of the box. No protective wrap or other protection. The bikes were packed this way on a return trip from Europe. They arrived late, but in good shape. We were a little concerned, as we had another 125 miles to ride from the airport to our home.

The Portland, Oregon Airport has a bike work station on the lower level of the airport. Other airports have similar place where bikes can be packed and unpacked.