Some of those gone, some in my lifetime, include,
The Riverside route from Heaton to Percy Main.
The Leamside Line from Pelaw to Tursdale Jcn. (been on HST's during diversions.) Not to mention the line linking Washington and Chester-le-Street and the line from Washington to Boldon Colliery
The line to Blaydon on the north bank of the Tyne from Forth Street following Scotswood Rd and crossing the Tyne just west of Scotswood Bridge. (Seen HST's on that during the Penmanshiel closure)
Diverging at Scotswood there was also a line through Scotswood tunnel through Lemington, Newburn to Stella North Power Station (now long gone), and orignally from Newburn to west of Wylam years and years ago.
The line from Monkseaton to Newsham, Blyth and Bedlington.
The line to Consett steel works from Durham
The line to Derwenthaugh coke works from Swalwell, (Originally ran to Consett too via Rowlands Gill)
Benton curve allowing through running from ECML onto Metro lines to Rowntree's at Fawdon and ICI at Callerton.
The BR line to South Shields ran on a different alignment between Shields and Tyne Dock.
The line from Tyne Commission Quay to Backworth
Also probably loads of colliery railways and the like in Co. Durham and Northumberland and loads more I missed or can't remember.
Have a look
here for a map of what used to be but be aware it includes waggonways, tramways and roads as well as railways. Closed railways are in
green and you can zoom in by clicking the map and navigate around by using the appropriate direction icon above or below the map.
Enjoy