The ORD ATS/train is operating between the terminals, but fortunately they still have the land-side inter-terminal buses on too.
Originally Posted by
imnotalawyer
This has been discussed in the
Wishlist/Speculation thread (a recap by
mia is actually the last post in that thread) but speculation comes down to there not being a suitable space available for rent.
IIRC it ultimately came down to Amex not being qualified as an airline or holding company with controlling stake in an airline. While such restrictions are not immutable, such a restriction has gotten in the way of others wanting to open a lounge at ORD.