A few days ago, David Llewellyn-Jones made the first port of OpenTTD to RISC OS.
OpenTTD is a popular open-source clone of the commercial game Transport Tycoon Deluxe, a Sim City-esque game where the goal is to build the most successful transport network across the game map.
To download a copy of the RISC OS source or executable of OpenTTD, go to this page on David's site. Unforutnately OpenTTD needs many of the data files from the commercial version in order to run, so you'll have to have a copy of that as well.
OpenTTD should run fine on an Iyonix (barring a few bugs which David is working on removing), and should also be compatible with earlier RISC OS machines. Without a copy of Transport Tycoon for me to test it with, I'll just have to guess that you'll need at least a StrongARM RiscPC to get it running at a playable speed.