The Planning Application can be found here
Mainly it's to modify the arches on the east side of the bridge for a new siding for a wash plant capable of taking a 12-car 700 or a 10-car 720, plus access for the Chisholm Trail.
Trouble is that whilst many indeed would see closing Mill Road to motor traffic as a good thing, Network Rail want 8 weeks of total closure, saying that the Carter Bridge is close enough for a pedestrian and cycling diversion- in reality it's over 1km diversion, which is significant for pedestrians.
The traders though only seem to care about motor traffic, which is odd given that between Coleridge Road and East Road there are all of six parking spots on Mill Road, all on the east side of the bridge- trade from passing motorists can hardly be that significant, though of course taxi and private hire drivers tend not to believe that double yellow lines or double yellow kerb marks (ie a Loading ban- or single yellow for timed loading bans) apply to them.