According to Southeastern website a non HS1 ticket with destination as London terminals is valid on any reasonable route to;
Victoria, Cannon Street, Charing Cross, Blackfriars, City Thameslink, London Bridge, Waterloo East, Waterloo mainline and Vauxhal.
Why do they include Waterloo and Vauxhal and is my ticket actually valid on SouthWestern Railway between these stations or beyond to Clapham Junction as a reasonable route to Vauxhal??
The system of 'reasonable routes' applied during the British Rail era, so it finished 23 years ago! Nowadays, it only applies when there's significant disruption and you are told that tickets are accepted on "any reasonable route".
Nowadays we have the Routeing Guide. This defines the routes that a ticket can be used on. Subject to any restrictions printed on your ticket (e.g. "Southeastern only", "not via HS1"), that's:
- The shortest route which can be completed wholly by rail (i.e. without walking from one station to another), as well as any route no more than 3 miles longer than this shortest route.
- Any direct train from the printed origin to the printed destination on the ticket.
- The "mapped routes" that the Routeing Guide permits.
The mapped routes are probably the most complicated part of the Routeing Guide, but as a whole, they
generally define those routes which are reasonable but aren't captured by the first two rules of validity (e.g. because a change of trains or stations is involved and/or it's a longer but faster route).
It's not possible to definitively answer your question without knowing your ticket's origin and route restriction (if any), but the rule of thumb is that you can go to any of the south-of-London Termini from origins south of the Thames. If your ticket is valid between Clapham Junction and Victoria then it is also likely to be valid between Clapham Junction and Waterloo.
