There used to be a Tyne and Wear Transport discussion group, not sure if it still going since the demise of MSNgroups. They used to have a bit of info on their site.
I'm sure you know anyway but distances used to be stuck or painted on each OHL mast and measured from the datum which is the middle of the junction at South Gosforth where the Airport and Coast lines diverge. The distances are all metric. The first 2 or 3 letters indicate the electrical section the mast is in, the middle set of numbers is the whole number of km from the South Gosforth datum and the last 3 numbers is the number of metres. Hope that makes some sort of sense.