I have been searching for info. on taking a train from Amsterdam to get on a half day Rhine tour from St. Goar to Bacharach and then return to Amsterdam, all in the same day... Is this possible and do I need advance registration...

Doesn't sound like a good time to me... you'll spend about 6-7 hours on a train to do a couple hours on the boat. The Rhine is very nice but it is not 'spectacular' scenery. The whole point of being in this area is to relax and enjoy small villages along the Rhine, take a chair lift or two, visit a castle or two, have a beer or two... The cruise is another activity that is fun to do, but I wouldn't consider it a 'destination' activity.

You can check train schedules on this easy-to-use site:
http://reiseauskunft.bahn.de/bin/query.exe/en?

Leaving Ams-Centraal at 8:05am, you'd arrive in St Goar at 12:19, as an example...

You MIGHT be able to go to Koblenz and take one of the boats a ways from there. It is a quite a ways from Ams to St. Goar or Bacharach - not really a day trip IMO. I think Koblenz is where some of the ships go out from but not sure that is really the most scenic part of the river. I would suggest trying to spend a nite or two in one of the small towns. It is a beautiful area, watching the boats on the Rhine is fascinating and it is a great contrast to busy Amsterdam!

We did this once from Hannover but we took the train the day before to Mainz and stayed overnight and then took the boat the next morning as far as St. Goar, got on the train and were back in the hotel late that afternoon/evening.