Is there a train that makes the tourist or historic route of the most important places in Wales?
If you want a train trip you must take the Ffestiniog railway. Its a small gauge mountain railway that was actually once used to work the slate mines. It's really lovely, particularly if it doesn't rain; which you'll be pretty lucky to get away with.
Some of the best places in Wales are a bit out of the way; they need a bit of effort to get to i.e. Lundy Island (and Tenby), Three Cliffs Bay on the Gower Coast, St Davids, Port Meirion ... about 7 years ago