Both or (n)either

While travelling to visit my dear old mum by tube over the weekend I noticed a poster warning that the Circle Line was closed for repairs so "no trains will be travelling in both directions".

Which appears to mean they have one set of trains running clockwise and another widdershins. Yes, I know, the poster's author meant "either" rather than "both", but why didn't he just say "The Circle Line is closed for repairs"?

Pedantic? Certainly. But remember, this isn't a slip of the tongue - it's a publication from a major company and it shouldn't be that hard to use the right word for the job.