I've walked along that section of abandoned line, and you could squeeze a double track through, although it would pass uncomfortably close to the building. Compulsory purchase the block, rebuild the line, then sell the flats again at a slightly reduced price.

However, Copthall only sees a match once a fortnight and seems to be coping with free shutt buses from Mill Hill Broadway. If you wanted to go all the way to Edgware, you've got the M1 and an industrial estate on the track bed, and they're just in the process of building a small housing development immediately west of the A1/A41.