"Nine Elms" is of course near the new American Embassy in London , such has the area changed.
Basically , the Post Office had access to all passenger trains through the UK for the carriage of mail bags , but there were special mail trains which were solely for their use and often conveyed special sorting vehicles. Parcels trains conveyed general (what you would call "express packages ?) , but might also have carried overspill mail loads. There were often huge quantities of mailbags carried , especially at high season times.
I commend to you a recent book by Graham Satchwell - a very senior retired British Transport Police Officer , who started as a constable within Southampton Docks - "Great Train Robbery Confidential" - ISBN 978 07509 9232 9 - published 2019 - who describes in some detail the port handling arrangements for bullion , - see especially Chapter 16 where options for robbing bullion were explored , and discounted for a raid on a postal /mail train. (£9.99 off Amazon or preferably another bookseller) - he has another book on his career , which goes into some detail on Soton Docks - "An Inspector Recalls - memoirs of a railway detective" - 2016 - ISBN 978 0 7509 6640 5
Excellent material , and superb underpinning knowledge. Do read them. (if you can)