In London they have a problem with ports. If I wanted to take a train to the coast it would seem sensible to take the train from one of the giant terminal stations like liverpool street but it usually works out better to take the train further down the line. If I wanted to take a train to southend on sea, I would take the C2C train from barking, not fenchurch or liverpool street because the tfl zone is cheaper than the commuter prices. The TFL oyster zone is always cheaper than the commuter train prices so you would want to use the zone as long as possible.
