On 9 Jul, 13:59, wrote:
My son bought a cheap day return ticket yesterday from Epsom to
Potter's Bar. He bought it from the Southern machine at Epsom
station. The outward journey (Epsom-Vauxhall-Finsbury Pk-Potter's
Bar) was uneventful, but on the way back, the barrier at Vauxhall
Underground would not let him out. He showed his ticket to the
attendant, who said it was not valid for the tube journey -- though
she let him out without asking him to pay.

Have just found this at http://www.nationalrail.co.uk/
stations_destinations/connections/crossing_london.html:

"Rail tickets for journeys routed via London are valid for transfer by
Underground or Thameslink between London terminals at no additional
cost. For example, a Brighton to Leeds ticket is valid on London
Underground between Victoria and Kings Cross."

I don't think his ticket actually said 'via London', but I can't see
how else one could go (other than via Guildford, Reading, Coventry,
Leicester and Peterborough, which is undoubtedly not a permitted
route). Granted he could have used the (much slower) Thameslink
route, but then the example they give - Brighton to Leeds - could do
so too, and perhaps even more plausibly so.

The ticket seems therefore to have been printed and coded contrary to
what it says on the website. Whether it was also priced wrongly is
also an interesting question. A CDR from Ewell East to Hadley Wood -
just one station inwards from each endpoint, but both in zone 6 - is
£6.70 (without railcard) as opposed to £14.40 for Epsom to Potter's
Bar (both taken from the NR website).

I also note the curious statement that "Please be aware, you may need
to buy separate tickets for this journey, as a through ticket may not
be available." All in all, a bit of a mess.

PeterCS