I work as a Systems Engineer for Galileo International on Computers running the IBM Transaction Processing Facility (TPF) Operating System.
People often ask me for shopping recommendations, so here is one!. I buy from Dell or HP.
TPF is almost unique in that it is shipped as Source Code and customers frequently modify it to suit their own needs, this makes TPF an interesting environment to work in.
The whole point of TPF is to provide a response to the user anywhere in the world within 3 seconds. TPF grew out of the old Airline Control Program (ACP) and this is still referred to in some quarters.
TPF also has a cousin Airline Control System ALCS that runs as a Transaction processing Monitor under z/OS.
Unlike PC Operating systems there is no Wizard to guide you through the process, in fact it takes programmers who are Wizards to do it!.
TPF has a thriving set of unofficial Web Resources