What a great subject not having been able to see the heyday of the railways I'm fasinated by dissused railway lines and old structures, nothing better to pass the time and try and follow an old railway line on google earth. Passing under the bridge of the great central at Finmere station does it for me espically when I catch a glimpse of the rolling stock in storage there I always wonder what it would have been like to see the Master Cutler flying over the bridge.
Agreed loved seeing the old shed at Bedford when I used to catch the train up to London there, Bedford was/is a good spot for old structures, the orginal St John's platform still remains (last time I was there it did) I think the old signal box on the Bedford to Bletchley line has gone but I used to love seeing that as a kid, there is majority of structures and old infurstructure left form the Bedford to Hitchin line, including Cardington Station and signal (although could have been put up by owners). There still is a bridge over the ouse for the line to Goldington power station and still a good bit of Potten station (a garden centre now I think) on the Oxford to Cambridge line.
Another great find when living in Cholsey was the track bed and overbridge of the Didcot, Newbury and Southampton line near Blewbury and the one of Bridges that remain futher along near East Ilsley. Now living in Faringdon I can see the old station from my Bedroom window, although now a nursery rather than derlict but I do wonder what it would have been like if the preservation movement had got there first. And of course there's the remains of the Highworth and Fairford branches that I see quite often on my travels around south Oxfordshire.