"Neil Williams" wrote

The difference is the infrastructure, not the trains. The Bakerloo
"shared" bit is +660 outer, 0 inner, so works for both. The Tube proper
including the Met is +440 outer, -220 inner, so no good for third rail
EMUs. Or something like that.

When the Croxley Link is built the section through Watford High Street will
have to have an operative 4th rail reinstated (for the Met trains) but at 0V
(bonded to the running rails for the LO trains. Should there be any
intention to run LO trains on to the existing Met (e.g. a Watford Junction
to Chesham or Amersham service) the Met infrastructure would have to be
altered in the same way as Queens Park to Harrow & W, Putney Bridge to
Wimbledon, and Gunnersbury - Richmond. It would be easier to use LU trains.

IMHO there is a case for the Croxley Link also to be used by trains from
Chesham or Amersham. But I doubt that there's a business case for procuring
new trains for this, and the likely traffic won't justify 8-car trains.
Perhaps a few of the D78 trains could be arranged as 4-car sets and modified
as necessary for future Met signalling. Certainly a better place to use them
than Harrogate.

Peter