Morden to Sutton - still possible?

Just noticed on Wikipedia that originally it was hoped to take the Northern
Line beyond Morden and onto what's now the Thameslink loop to run down to
Sutton - see http://en.wikipedia.org/wiki/Morden_South_railway_station

How far apart are the lines now and what would, other than line capacity,
carriage size and cost, would prevent such a connection being made today?


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