[series vs parallel for DC traction power]
That's beyond my knowledge...

It's very simple: idea is to not use "half the batteries" at half power
but to use all the batteries.

Traction motors had several windings which could be connected in series
(for low speeds) or in parallel (for high speeds) depending on the
application.

In a battery application with one winding you'd bank the batteries to
be in series or parallel based upon need.

But that's mostly historical... all DC traction power applications now
use "smart" electronics to drive AC motors (often 3 phase... sometimes
more!).

Tim.