Eventually found a definition of Northern Powerhouse rail Core in a House of Commons Library document, dated 25th August 2022
Good. That's more or less what was in the Intergrated Rail Plan (page 100).
"Northern Powerhouse Rail:
what the Government proposes
in the IRP core pipeline:
• between Liverpool and York, to build NPR in line
with the 2019 Option 1 developed by Transport for
the North. This will see:
• 40 miles of newbuild high speed line between
Warrington, Manchester and Yorkshire (to the east of
Standedge tunnels);
• upgraded and electrified conventional line for the rest
of the route;
• significant improvements to the previous
Transpennine Route Upgrade (TRU) plans between
Manchester and Leeds, including electrification of the
whole route, digital signalling throughout, significantly
longer sections of three and four-tracking, and
gauge upgrades to allow intermodal container freight
services. This will now form the first phase of NPR;
• electrification of Leeds–York with some sections of
four-tracking;
• upgrades and electrification of the Leeds–Bradford
section of the Calder Valley Line; and
• reinstatement of Warrington Bank Quay low level
station; upgrading and electrifying existing lines
between Warrington and Liverpool; and enhancing
Liverpool Lime Street station. "