The Sodor and Mainland Railway originally attempted to cut a tunnel through the Ballahoo Ridge. However, it collapsed during construction, resulting in the railway only running trains between Ballahoo and Kirk Ronan.
After its foundation in 1915, the North Western Railway constructed its own tunnel in order to reach Vicarstown. The NWR was built as a strategic railway in a time of emergency. Speed and cheapness of construction were paramount, so a single line bore only was cut, with the Up and Down lines being gauntletted. While work was in progress, a section in the centre collapsed, leaving two tunnels (the Western - 1 mile, and the Eastern, or Henry’s - 450 yards).
The tunnel gained its nickname in 1922 after Henry refused to come out, claiming that the rain would spoil his "lovely green paint with red stripes", and was subsequently bricked up after three failed attempts to get him out. A second bore was cut alongside to provide a double line and Henry was eventually let out sometime in 1923 to help with the express. In 2011, the tunnel collapsed but was repaired in time for the unveiling of the Thin Clergyman's bust at Tidmouth.
In the first series of the television series, Henry stopped in the tunnel (like in the Railway Series). In the fourth series, the tunnel was blocked by an elephant and Henry had to take some workmen to clear it. In the thirteenth series, while painted pink, James hid in the tunnel. In the nineteenth series, Thomas hid in the tunnel from the Fat Controller after lying about an accident.
- In Wilbert Awdry's original drawing, the tunnel had one track instead of three.
- Henry's Tunnel has had numerous modifications throughout the years:
- Series 1:
- One tunnel bore had two tracks and the other bore had one track.
- Series 4:
- The numerous switch tracks and the siding disappeared.
- Series 7:
- Series 8:
- Both bores had two tracks.
- Series 13:
- The tunnel seemed to get shorter, since its other end was visible.
- Blue Mountain Mystery (only):
- The tunnel was seen in a flashback and it had one track in each bore.
- The Adventure Begins:
- Two switch tracks were added in front of the signal box.
- The tracks became slightly curved on the opposite of the tunnel.
- Series 1: