Still, freeware is not open source. There are GPL clones of RT. The two most notable ones are Railz and JFreeRails. Indeed JFreeRails got updated as recently as September this year. Railz seems to have stalled with it's development.
Then there is of course the other rail game implementations, notably OpenTTD. There are lots of promising noises coming out of the OpenTTD camp and the hires graphics seem to be coming out of the community at a steady pace. I'm very hopeful that we will see a hires release in 2007 that should be a superb game and herald a new era for OpenTTD.
A game that already has hires graphics and is free, albeit only freeware, is Simutrans. The development is relentless and the latest release looks very nice. For some reason I have never found Simutrans as much fun as OpenTTD but then I grew up with the original Transport Tycoon.