is it friendly with Trainz: Yes
Is it a good buy: No
The ATI HD 2900XT is a notoriously hot card. Core temps run well above 70C 24/7. If you want a new video card go for an HD3850 or HD3870
ATI will also be releasing the HD3950, HD4850, and HD4870 within the comming months.
Asfor OpenGL mode: ATI has never done that well with OGL. They will work, but you'll see better performance running Trainz in Direct X mode.