The liftout rests on the bottom part of where the door jam supports were cut-out to allow the train to pass through the doorway. They are about 5/8" wide, but only extend out about 4". That's fine for the left side of the liftout, so it sits right on the door jam support. See pics attached.
The right side sits on the door jam support but since the right side of the liftout is about 12" wide, that would not have been enough support and to insure correct alignment.
I don't have a picture of it, but I have already added a cleat to the side of the wedge extending from the end of the door jam support to the end of the liftout at it's widest point. In addition to extra support, I also needed something to drill into to install the pins and dowels on that end to keep the liftout in alignment.
The cleat had to be screwed into the side of the wedge and not underneath because the existing shelving is about a 1/2" off from level across the door. That also meant the existing right-side door jam support at that end of the liftout had to be cut 1/2" higher than on the left to get the liftout level.
I'll post some more pics which will make it clearer, but you can also see it from the attached sketches.
I think Jan and Carl are right, too - I just always pictured a liftout as needing to be cut right at the edges and never had the foresight to think out of the box that you could eliminate any cutting by just disassembling the track on either side of the liftout - it takes a village !! 
