Depending on your resolution, you probably be able to run with anti-aliasing (AA) and anisotropic filtering (AF) and it will still be much faster than your 8400 GT. If you run Trainz at 1680 x 1050 or less, then the 4670 will definitely be faster even with AA & AF. At 1920 x 1080 & higher, the increased performance would not be noticed as much without lowering settings slightly.
Hope this helps.
Kyle