When I've used grain of wheat bulbs, I've found that they're very white at 12 volts and don't last too long. I've generally run them in series pairs from a 16 volt supply, so each bulb gets 8 volts, they 'look' much better, not a sharp white, and last for ever. The only down-side is that if one bulb fails the other will go out, too - a sort of mini-Christmas-tree-effect - but, I've never lost one at 8 volts..
With tungsten bulbs, you can run them off AC - I've used spare Hornby power supply transformers, so that they're separate from the track supplies.