overnight train trip to the Loire Valley from Paris

Catch the train in Paris Gare de Austerlitz to Amboise.
It only takes a couple of hours on the train not overnight.

Are you talking about an overnight train trip or an overnight trip to the Loire Valley? Are you planning to rent a car while in the Loire? I usually go to Tours or St-Pierre-des-Corps when I am taking the train to the Loire Valley -- Tours if I will not have a car and St-Pierre if I intend to rent one while there.

You can also take a high speed train into Tours. Takes about an hour. There are car rental companies at the Tours station.

Catch the TGV from Paris Montparnasse to Saint Pierre des Corps or Tours. It's a shade over one hour (at 186 mph) and you stand a better chance of picking up either a tour or a hire car. Tours is the regional capital and transport hub, so there are a number of chateaux you can visit by public transport.

I know RS books concentrate on Amboise, but it's a small country town, the station is a 20 minute walk from the chateau, and the only car hire place in town is at least 45 minutes on foot from the station. It's not a big concession, either, and most of the vehicles are utilities.

If you're thinking of staying the night in the Loire Valley Tours makes the most sense - more hotels, better restaurants, and better transport links. Staying in Amboise means that to visit any chateau except Amboise by public transport you will have to travel via either Tours or Blois