Nothing to do with network rail. These issues were caused back in 1988 when BR strung the knitting up to a price point. At the time the electrical capacity required was a lot less than is now being demanded. At the time of electrification there was only one Electra in each direction per hour plus potentially one freight/parcels loco as all other EC and XC services were loco hauled or HST. Now you have up to 2 LNER 80x plus one TPE 80x and a ScotRail 380 or 385 potentially in that same electrical section at the same time all making power demands. The main issue with Marshall Meadows is the lack of National Grid infrastructure in the area that can boost the electrical supply with so is not exactly a small job. Hutton Bonville also has a lower output however there are other NG feeds that can reroute and boost the power much easier. So until these works are complete LNER are only supposed to be running max 9 car units to Edinburgh and TPE are diesel operated between Morpeth and Edinburgh station stops (we do not change power on the move) in addition TPE are diesel mode between Durham and York on Sundays on account of LNER running more Anglo Scottish services on a sunday.
One root cause could be said to be down to BR (well, the contemporaneous Treasury if you really want to dig down into it!).
But I think this mainly comes down to a lack of co-ordination across the rail industry, during the period between the decision to introduce extra electric trains/services, and when those services were actually introduced. Power supply upgrades inevitably take ages, so prevarication isn't going to help anyone.
Easy to say with hindsight of course.
