Not the sort of MUSIC you can dance to, but nevertheless a small discovery, at least for me. On a trail that started from a YouTube clip on CTSS - a contemparary report by a young Fernando José "Corby" Corbato, leading to different Wikipedia articles on timesharing, I found MUSIC/SP amongst the TSS, MTS and TSO systems, a product of McGill University in Canada and once an IBM product - on of the timesharing systems for its mainframes. You can read about it at here. It is based on an earlier IBM timesharing system called RAX, of which hardly a trace is to be found on the net.
A working system is downloadable at here - it is runnable under a windows based simulator or under the multiplatform Hercules mainframe simulator. Being on a Mac, this is what I did. It offers a wealth in programming options and uses an interface that is quick to be grasped - including an ISPF/PDF like editor and panel system. You will need Hercules and a 3270TN Terminal Emulator. The system's console is a Telnet session, but the user terminals require the 3270 protocol. A Hercules configuration file is included, so you are good to go.
This whole menuing system is written in Rexx. When running CPS - the venerable clauses-per-second benchmark, I noticed that the system announces itself to "parse source" as CMS. Some inquiries later it became clear that the original VM Rexx interpreter code is part of MUSIC/SP. So everyone who has an interest in checking out the VM Interpreter can do so now using a legal download - most publicly available VM releases predate the release of Rexx and are a such of less interest to the Rexx community.
Rexx and other programming languages are runnable with a scheme that requires hardly any JCL - in fact none at all if you put the required magic incantion on the top of your script files. It is a nice environment for experimentation, and it is very laudable that these products have been preserved for posterity. A Rexx class, using the real interpreter, could be held in the cloud without any setup using IBM's (Rational now) 3270 on demand product which runs in the browser.
If you want to set up a MUSIC/SP system for yourself and you run into any trouble, just drop me a line.