As far as In know, SABRE still runs on System/360 today.
It was created in the 50's, migrated to System/360 in the 70's, and is still the central system where all bookings of all airplanes are registered, and where travel agents from everywhere in the world can check for availability, make reservations, etc.
The main interface is a console. I've seen several attempts to create GUI's but they can't fully replace the efficient text-based interface that exists to enter commands and see results.
If you ever fly on a plane, your booking went through that system, no matter where on this planet you live, or where you went.
There's an impressive number of transactions going on in that system, with lots of clients and lots of data in there.