The trowel is 32 cm long so the slab needs to be at least 27 cm x 19 cm, or about a foot square. Unfortunately the slabs in our garden are much bigger.
The remains of the timber that once covered the hole are underneath the cycle track top layer. A fragment of the rotted wood can be seen in the second picture.
The timber must have been covered by ballast when the railway was operating, and the inspection chamber presumably only opened up whenever there was a blockage. The concrete pipe was probably a later addition following failure of the much larger clay pipe lining at some point. It would be useful to know if the chamber was built to improve the chances of clearing flood water from the station area and whether there are any more along the line; subsequently covered during the Greenway construction. The concrete pipe seems to be the same size as the one that drains from the trackbed into the stream close to Bannell's Lane. Incidently, on Friday (21 Dec) the pipe near Bannell's Lane was sending several litres per second into the stream, so if a partial blockage is causing the "blow-holes" in this area the potential flow must be considerably greater. Note very little water flowing through the pipe at Etwall Station. The electric cable crossing the under the trackbed is intriguing. Can anyone remember whether Etwall Station was converted to electric lighting before closure?
2nd January 2013 This hole has been made safe by capping it with a stone slab.