How do I get there?
King's Lynn, Watlington and Downham Market on the west of the loop are served by direct trains from London King's Cross which don't require reservations but also require most bikes to be left in only a couple of multifunction spaces or the unused doorways which can mean moving them around during the journey. Changing at Ely connects these to trains from the midlands. I recommend using King's Lynn station so that the ride starts with the famous route to Sandringham and finishes with a great view of the town.
On the south of the loop, Diss is served by direct trains from London Liverpool Street towards Norwich which require reservations but have recently been upgraded to have six spaces per train and level access from platforms.
Cromer, Great Yarmouth and Lowestoft have trains from Norwich, while Thetford and Brandon can be reached from both Norwich and Ely. Most of these use regional trains that are a shorter bi-mode version of the London trains that still have six bike spaces, but do not offer reservations. These services may be more useful if you have to cut your trip short, rather than as a planned start/end point.