John B wrote:
...
IANAelectrical engineer, but I just don't understand why it it
would
make economic sense to switch to traction types that are
inherently
less efficient.

Capital vs. Revenue is one reason I can think of. The financial
model for train operators must be very complicated - so a simple
Net Present Value and Payback Period calculation may not be
possible. The model used may show that in the short-to-medium
term, it's more profitable to use AC. Long-term savings aren't
relevant to a franchise that may only be seven years long, if the
savings aren't realised until after year 15, for example.

--
MatSav