Flippin 'eck - I was just goin on about that mapperley tunnel to some work mates the other day. I used to play down there as a kid, at the east portal, in the mid 80's. I never knew where it came out on the north end - now I do!
I'm sure you lot know this site already but I found it great to overlay a modern map onto an old 1920's map of the area, you can really pinpoint old railway lines and where they used to be. Check it out:
http://www.ponies.me.uk/maps/osmap.html
Just zoom into notts and away you go
