On 2016\09\02 20:04, Charles Ellson wrote:

Popping up tends to require a bit more excavation. That costs more
money and might in some places be undesirable anyway if only for the
need to ensure support for what is being burrowed under. It's probably
a bit late now anyway.

A virtual cross-platform interchange could be created by having several
lifts along the platforms that go up, over and down. I doubt that the
BCR would be favourable, and yet the cost must be a tiny fraction of the
expense which was incurred on building multiple real cross-platform
interchanges into the Victoria Line.