The proposed underground line will be linked and bundled in with United's new stadium and the redevelopment of the surrounding area, includingTrafford, Trafford Wharf, etc.
If there is only 1 city centre Underground line, it will run from the New Old Trafford, to Victoria Train Station, to the new underground train station at Piccadilly Train Station, on to the Etihad Campus. Ultimately it will free up space and take passengers off Metrolink. Running the underground line to both stadiums will make a shit load of money for the underground. 62,000 fans at the Etihad, and 100,000 fans at the new Old Trafford.
That's what I think.