Tidmouth Tunnel

The Castle Tunnel is a tunnel on the main line. It first appeared in the third season and was last seen in the ninth season. It was originally on the Little Western, but it seemed to be moved to the main line in later seasons. In the fourth season, it was on Thomas' Branch Line. Although never mentioned by name in an episode, it was first named in an eighth season learning segment. It will return in Tale of the Brave.

Trivia

 * The lake next to tunnel shrunk in the later seasons until it was no longer visible. It became visible again in Tale of the Brave.
 * The number of tracks leading through the tunnel changes several times: sometimes there are two tracks, sometimes there is one track, and sometimes there are three.