Old Warden Tunnel

Old Warden Tunnel is an abandoned railway tunnel near the village of Old Warden in Bedfordshire.
https://www.abandonedengland.com/oldwardentunnel
History.
The tunnel was built as part of the Midland Railway connecting Bedford and Hitchin, and from there to London between 1853 and 1857. Contrary to popular belief the line lost its passenger services before the cuts of Dr. Beeching, they were withdrawn on 1 January 1962 along with goods services between Hitchin and Shefford. Goods services between Shefford and Bedford succumbed to the Beeching Axe on 28 December 1964. Built of blue engineering bricks, the tunnel is ovoid in shape and runs at a very slight gradient to allow drainage; however, it is perfectly straight. 
The tunnel has no blast relief ducts, due to its lack of curves and relatively short length, allowing good air passage. Regular niches were cut into the wall to allow maintenance on the permanent way during running hours. Finally, both portals were capped with stone and it covers a total length of 882 yards.
The tunnel today.
The tunnel has lain derelict for over 40 years, but is in fair condition, with the Hitchin end back-filled to within 7 feet of the tunnel roof and the Bedford end bricked up with gratings for local bats. However, public access holes have been closed at both ends. Entrance to the tunnel is not recommended, due to bats and standing water accumulation. The Bedford Portal is still visible in its cutting, but the Hitchin Portal is entirely covered in undergrowth.
Nature reserve.
The land above the tunnel is Old Warden Tunnel nature reserve, which is managed by the Wildlife Trust for Bedfordshire, Cambridgeshire and Northamptonshire.
The tunnel in fiction.
The tunnel makes a brief appearance in Those Magnificent Men in Their Flying Machines. This scene was filmed shortly after the tunnel had been closed to traffic, before the rails were lifted. The now-demolished Bedford Power Station can be seen in the far distance.
References.
https://www.abandonedengland.com/oldwardentunnel