I come back to this - how about a more progressive idea?
Overhead electrification on all the simple places (most of the network), with gaps in the expensive to wire places. Stadler EMU's with backup batteries enough to carry them through the gaps. Recharge while under wire.
Why oh why is this not being pushed? Would be far less expensive, but just as effective as full electrification. Huge batteries would not be needed to cover 2-3 mile at most gaps.