In message , at 09:01:32
on Mon, 11 Aug 2014, remarked:
From the context, and the fact they've been awarded a temporary
extension while the DfT gets its act together for new tendering, I
suggest it means "Short Term Agreement".

See also:

http://www.railwaygazette.com/news/p.../greater-angli
a-direct-award-franchise-agreed.html

Which does nothing to answer the question.

"The short-term agreement features a £20m package of improvements
which includes..."

Your surmise then. No use of or decoding of the acronym.

It's the only meaning that makes any sense. Especially as Abellio are
crowing (in their stunningly inaccessible e-magazine) about how they
have several unprecedented things happening in the STA period which map
closely to those mentioned in the article.
--
Roland Perry