The reason for Newcastle's layout is historical - the High Level was built first in the era of Queen Victoria and had a triangle junction at the south end. This fed into the east end of the station, meaning that anything coming from the south and heading for Edinburgh via what is now the ECML would have to reverse. The west end fed out onto what is now the Forth Branch, but used to run through to Carlisle (joined the current route from Blaydon).
The current ECML approach from the south is over the more westerly King Edward bridge, named after the reigning monarch at the time of construction, enabling trains from London to come in from the west and head for Edinburgh via Berwick without reversing. It also has a triangle junction at the other end; both these triangle junctions are linked and so a train can approach from south of the river, go over one bridge, through Newcastle station then over the other bridge and come back the other way without reversing.