Request for comments on AC mains frequency conversion method - Supply>PSU>AutoInverter>Load
Does the train set need 60Hz? Most electronics will handle the small difference between 50 and 60Hz. If the input transformer has been too agressively designed, then it may need some input voltage derating from nominal.

If 60Hz is needed, then PC supply to inverter sounds as good as any other way of doing it. It uses low cost commercial components. Be aware that some PC supplies need a minimum load on 5v for their other rails to stay in regulation. A 50w 12v halogen that draws a couple of amps at 5v is my preferred choice of dummy load.