I've had mine set to 512MB for a long time. My system has 3GB RAM, and a RAID0 HDD system.
I've tried setting the cache size to 1GB, but it didn't do anything.
Maybe I should try going back to 256MB and see whether anything changes for the better.
I have a large route with a lot of assets, but I am trying to use many of the same ones so it doesn't slow the game down too much.
Damn; I hate referring to Trainz as a game. But it just sounded better in this context<g>
FW