Interesting - do you have any references/links I could pursue?
I'd be interested in hearing more about this too - it's a new one for me. March and Woodford Halse sound like a lot of wires, gantries and electricity substations beyond Sheffield.
It's not an area I know much about, but I thought the next on the LNER's main-line electrification hit list after Woodhead was something like the ECML between Newcastle and York - before WW2 intervened and British Railways came into being.
Regarding the Woodhead scheme itself, there was a planned but cancelled "Phase 4", involving continuing electrification around the Fallowfield Loop into Manchester Central (for passengers) and to Trafford Park (for freight). This was cancelled beyond Reddish Depot as part of an early-1950s economy drive.
I read somewhere (can't recall where
*) that prior to that 1950s cost-cutting, there had also been an aspiration to continue the DC wires from Manchester Central along the CLC route into Liverpool Central High Level - presumably also serving the Garston area and South Liverpool Docks for freight.
Maybe this hoped-for Liverpool Extension was the reason for the initial proposal for a larger number of EM2s? Could there have been a plan back in the day for regular EM2 loco-hauled expresses between Manchester Central and Liverpool Central, independent to the Sheffield services?
Another factor is, around the time of World War 2, railway companies would have had around 100 years of experience in working out how many
steam engines were needed to operate a certain level of service, but no-one had real-life experience of how much more efficiently a fleet of main-line electric (or eventually diesel) locomotives could work the same number of daily trains. Maybe the relevant committee in HQ said something like: "We would need X steam locos to work hourly expresses between Manchester and Sheffield. Electrics will be more efficient, so we'll knock off - oh, let's say 15% - from that number. What does that come to Jenkins?" "29, sir."
[EDIT]
* It was a post in a Woodhead-related thread on RMWeb
here.
According to subsequent replies in that thread, the CLC Liverpool Extension was mentioned in an early edition of
Backtrack magazine and in the Foxline book
Woodhead - The Electric Railway.