I've tried just about everything short of a new system, and cannot run the more complicated routes with more than one train in the frame without dropping way down to 7 or lower.
I can get better rates when I reduce the view distance, the scenery detail (tree detail in TS12), and yes, by reducing screen resolution.
I run TS12 on an old (6-1/2yrs) system
3.4Ghz single core processor
3Gigs DDR2 530MHZ RAM
NVidia GeForce 9800 GTX+ 500MB
Total of about 1TB HDD, in 4 drives, SATA.
The drive TS is installed on has 32MB cache RAM.
The biggest issue I have is trying to run multiple trains in congested areas. I have created several routes with large yards, and it is nearly impossible to fill the yard even half capacity and still play the game.
I would love to get a new system, but just don't have the cash. At this point I don't consider upgrading any more components, as it is the processor and memory that are the bottleneck.
One thing I have read though, is that installing TS on a RAID (striped array) configuration can help the stuttering problem.
In the mean time, just keep all your routes on the simple side. Desert or plains, not forests. No cityscapes, except low-res backdrops.
FW