"Henry's Tunnel" is the nickname given to the tunnel on the Main Line near Ballahoo. It got its name in 1922 after Henry refused to come out, claiming the rain would spoil his "lovely green paint with red stripes" and was subsequently bricked up. A second tunnel was bored alongside and Henry was eventually let out sometime before 1923 to help with the express. The tunnel later collapsed but was repaired in time for the unveiling of the Thin Clergyman's bust at Tidmouth.
In the first season of the television series, Henry stopped in the tunnel (like in the Railway Series). In the fourth season, an elephant blocked the tunnel and Henry had to take workmen to clear it. In the thirteenth season, while painted pink; James hid in the tunnel. In the nineteenth season, Thomas hid from Sir Topham Hatt in the tunnel after he lied about an accident.
- The tunnel's television sets and CGI render have had numerous modifications throughout the years:
- In the first season, one tunnel bore had two tracks and the other bore had one track.
- In the fourth season onwards, the numerous switch tracks and the siding disappeared.
- In the seventh season, each bore had one track. A signal box and water tower has appeared and disappeared at the portal since the seventh season until the CGI switch over.
- From the eighth season onwards, both bores had two tracks.
- From the thirteenth season onwards, the tunnel appeared to get shorter as you could see through it.
- In Blue Mountain Mystery, the tunnel was seen in a flashback and it only had one track in each bore.
- From the Adventure Begins onwards, two switch tracks were added in front of the signal box and on the opposite side of the tunnel, the tracks became slightly curved.
- In The Great Race, a set of tracks are added above the tunnel portal.