Looking to get a train from Edinburgh up north or to Galloway to do some off road riding and then camp followed by some more riding. Dont have a car so needs to be accessible by train. Any suggestions?

i`d consider staying at the glenmore mountian lodge. nice quality hostel type accommodation and has loads of accessible trails round about. Bit safer bike storage overnight also. Its obvoiusly just outside glenmore which would be easily accessible by bike from Aviemore station.
They actually run MTB course s but would equally have loads of info and advice for cycling the local trails.

Spoiled for choice really - Aviemore is an obvious choice, Blair Atholl another. Depebnds on what type or riding you are after and whether you plan to wild camp en route?

Train to Kingussie and ride into Rothiemurchus, with Feshie and Rothie/Glenmore trails on your doorstep, everything from fun to epic.
Stay either at Ruidh Etchachan or Rothie bothies. Drier ground here than anywhere further west and wild campng is possible if you don't mind the midgies.

Get off at Corrour or get off at Rannoch station, more than enough there to keep you going.