Plus a section of the line going west from Mill Hill East towards Edgware is also a nice (woodland) walkFinsbury Park-Muswell Hill (Highgate section excepted) is the Parkland Walk (sadly).
Hayfield branch is listed there, it's known as the "Sett Valley Trail".To add a few more
Hayfield branch
I walked a bit of one at Dalmeny/South Queensferry this afternoon (no interpretative signboards to tell me what it was, but it was pretty obvious.
Brampton Valley Way - Northampton to Market Harborough.
Also part of the area around Stanwick Lakes uses the old Northampton to Peterboro alignment.
Fallowfield Loop in Manchester.
Parts of the Welwyn - Dunstable line - alongside the busway and also south from Luton towards Harpenden.
The Nickey Line, Harpenden to Hemel via Redbourn.
Already mentioned.Fallowfield Loop
These don't seem to be in the Wiki article:
The Cotgrave Colliery branch (Link to news article)
The West Bridgford GreenLine (Link to web site)
Killamarsh to Renishaw (both north east derbyshire).