It's not that it can't run on vista. TRS2006 was just released before vista was, so it wasn't listed as an OS on the box. Many people on this forum have TRS2006 running on vista.
I have been running 2006 on Vista since July last year and I have had no problems whatsoever. Some people seem to have lots of problems but I have had none. Obviously touching wood on saying this but everything has been fine.
However after installing 2006 and SP1, before launching, I have changed the bin applications to "run this program as an adminstrator", e.g. CCP, CCp Console, Content Manager, trainzutil, TPS2006. Some people have also changed other application files as well. Alan Yeomans gave a good summary in a posting on 30 April 2007 and I recommend that you look at this on the forum as it worked for me.
as long as you follow the installation tips here on the forum it runs perfectly fine. basically it needs to be installed outside the /Program Files/ folder and has to have "Read Only" unchecked.
After you have installed TRS2006 as a administrator on Microsoft Windows Vista:
Right Click the following files in the TRS2006 Directory below and select compatibility tab and tick the small square box ----> Run this program as an Administrator.
I'll add that 2006 also runs perfectly with Vista SP2 beta. I loaded it this past weekend and I've had zero problems. This is Vista Ultimate 64-bit by the way so whoever started a dumb rumor like this needs to try getting the program to run in DOS. Then they can say it won't load.