"MichaelJP" wrote in message
...

Perhaps someone in the know could answer this - given a blockage between
Doncaster and York why couldn't GNER simply route trains around the
problem
by going Doncaster-Leeds-York and vice versa?

Because Leeds - York is not electrified! For the distance involved it was a
stupid decision not to electrify through to Colton Junction when the
original electrification was carried out, although (as most of us know)
everything to do with the ECML electrification was 'on the cheap'.