wrote in message
...

But one is allowed to use Oystercards from Euston to Watford Junction on
Intercity, high-speed trains that call at the latter, are they not?

I think because they have 'set down' restrictions from Euston to Watford
Junction they can justify not accepting them.

[It's different if a boundary zone extension ticket is held to a destination
beyond WJ, I believe]

Paul S