In Belgium, province
Namur, commune Houyet the Hour-Havenne tunnel was part of the former
line 150 Tamines-Jemelle. The tunnel is about 500 meter long.
See for instance :
http://users.telenet.be/pk/lijnen.htm#150
http://nl.wikipedia.org/wiki/Spoorlijn_150
The East entrance of the tunnel ( N50° 10.353
E5° 02.519) has a stone with the year 1891 on it.
This section Houyet-Wanlin was opened on 1894-01-04.
Passenger traffic on this section was halted on 1959-04-05 and the
Houyet-Hour-Havenne section was broken up in 1978-1979.
It is now a walking and cycling path part of the
RAVEL
network of Wallonia.
The width of the tunnel and some bridge pillars over the Lesse river
seem to indicate that it should be a 2 track line, yet all sites
discussing this particular section mention only a single track.