Clue: Forth Rail express starts at York — train finally arriving, when not bound to be so? (8)
Answer: FREEBORN
Explanation: Definition: when not bound to be so
="when not bound to be so?". The starting letters of F[orth] R[ail] E[xpress], plus EBOR="York" (abbreviation of its Latin name Eboracum), plus [trai]N