"A train trip:" is just there to make a logical sentence, and is not part of the clue. James Bond's boss is M, and the rest of it is to be taken literally.
Time to kill this one. James Bond's boss, M, got ON, so that gives M-ON. A part of Eltham is ELT, and he got ON after seeing that. Therefore M-ELT-ON. And yes, Melton is a Metro station.