On many lines, London / long distance expresses call at the more major stations with local trains between them. In a very few cases, those local trains do NOT connect the more major stations at each end, and the Newport to Bristol Parkway stretch is a case in point; there is "nothing" (*) from Newport via intermediate stations to Bristol Parkway and that means that Severn Tunnel Junction, Pilning and Patchway don't even have a service to London via a connection at the next station towards London.