Swapping PSU's for a trial run won't break any warranty...there's no seal, so they don't know. I just don't think it's the PSU that needs swapped out since I don't think it's an issue. If the issue is Trainz only...I just highly doubt the power supply that feeds energy to the system is not compatible with a game.
The graphics on that is listed as being a ATI Radeon 3000 which should handle Trainz 12 just fine. The specs just blanket state "AMD Phenom II" and "up to 16G memory" which I'm sure is up to the buyer to configure. Depending on what you selected, a 300W PSU would still be find and not underpower any part of that desktop. So...the configuration chosen needs stated far before you start talking about tossing random parts at this thing. The PSU, since it's just one game having issues, being the last to blame. It could even be more simple as a game that has had no update since v1.0 and not hardware at all.
It could also be something as simple as game settings and expecting the best from onboard video. So give the config you have chosen, the screen it's trying to draw on, and what exactly is happening with the game (glitches, stalls, odd blocks) and we'll try to make you look good. You don't have to tell him you asked either; we don't know the guy! Crap...unless he read this.
And hey, don't worry about the old man. It took decades before I heard "good job". Decades after that, I was told "I've been watching you learn and was just being crucial...that's what Dad's do". And yup, this Dad's been doing it as well. /end sappy country song