Did you know that Mill Hill East terminus was originally to go to Mill Hill, Edgware East, Brockley Hill, Elstree South and ending at Bushey Heath.
The Edgware Terminus actually has abandoned tunnels into the Hill.
I think the Author was thinking more about the High Barnet terminus, West of Potters Bar, Colney Heath, between St Albans & Hatfield and north to join the old line round Wheathampsted, Batford and onto Luton, past Luton Parkway's door.
I know it will never happen but would pull a good number of cars off the roads down the M1 & A1M currently.