Surely organising train crew (notice in the crew room offering overtime?), organising paths with network rail etc is the sort of thing that the people running rail companies do all the time. This surely shouldn't take more than a week to organise and SWFC have presumably known their fixture list for months.
There seems to be a lack of can do attitude in terms of getting extra business sometimes.
DDB
I was more thinking of the fact that the question was asked of the next day.
I agree that with planning and a good business case it is possible. Re the specific example (Sheffield to Nottingham) it wouldn't be that practical for EMT to strengthen their trains (which are either Sheffield-London or Liverpool-Norwich) so their realistic option would be a shuttle.
In theory Northern could double up a couple of Leeds-Nottingham units. The difficulty here would be that the service of choice SHF-NOT probably wouldn't be the service of choice NOT-SHF so does the extra unit get worked back and a second extra unit sent to double up a different diagram, or do they look into leaving a set somewhere in Nottingham station for 3 hours - almost certainly requiring a second driver to take it off and shunt it out of the way somewhere.