Berlin: Grunewald, accessible by S-Bahn
Philadelphia: Wissahickon Valley, actually inside the city, with 21 miles of single-track in a wild river valley.
London: Epping Forest, accessible on the Underground; north Surrey hills, just under an hours drive.
Montpelier (France): Verdon Gorge, about an hours drive, and lots of nice stuff before you get there.
Chicago: yeah, ok. You got me.
Berlin: Grunewald, accessible by S-Bahn
Philadelphia: Wissahickon Valley, actually inside the city, with 21 miles of single-track in a wild river valley.
London: Epping Forest, accessible on the Underground; north Surrey hills, just under an hours drive.
Montpelier (France): Verdon Gorge, about an hours drive, and lots of nice stuff before you get there.
Chicago: yeah, ok. You got me.