The Catesby Tunnel is on the old Rugby-Woodford GWR line, just north of the village of Charwelton.
To the north there are five large ventilation shafts showing the line of the tunnel.
The co-ordinates are for the South Portal. I did not visit the North Portal, and the co-ordinates I have given for that are approximate.
The gate at at the South Portal was unlocked until recently, but is now padlocked.
There is limited public parking in front of the Fox and Hounds Inn in Charwelton. Or, if you pop into the pub for a drink, and ask nicely (as I did) they may allow you to use their carpark.
There are some old photos of trains passing through the tunnel
here and
here
There are some interesting photos of the construction of the tunnel
here
According to "Conserving the Railway Heritage" By Peter Burman & Michael Stratton it was opened in 1897 and closed in 1966.