They seem to be missing the point that no self respecting Geordie would ever venture into foreign parts in the first place. I'm sure the feeling is mutual.
Seriously, there doesn't seem to be especially vast use of the current railway link. I can't see everyone in Newcastle sudenly wanting to go to Middlesborough just because it's fast (beyond the new shiny thing crowd). The other problem is where to put the thing. The time it takes to get from A to B might become a rounding error, but it's not that convenient if you actually want to get from C to D, and have to use traditional modes of transport to get the rest of the way there.
They would therefore have to get a completely new, completely incompatible new line into the middle of Newcastle. About the only way I can think of doing this is to single the conventional line on the high level bridge (already only electrified on one side) and have a single line maglev into the station. Except that that would then conflict at the north of the bridge.