Ulfstead Mine, also referred to as Stephen's Mine, is an old mine that was first introduced in King of the Railway. It is located beneath Ulfstead Castle and Stephen became trapped in it after he had to bash through the blocked entrance to avoid a collision with some trucks. King Godred's long-lost crown, which was stolen a long time ago, was discovered here by Stephen.
In the eighteenth series episode, Missing Gator, the entrance to the mine was recently reboarded up until some runaway trucks bashed through the blocked entrance and into the mine where Percy came to rescue them. The bridge had also been rebuilt from having partially collapsed in King of the Railway, suggesting that it may have been briefly reopened during the time between the aforementioned special and episode.
In Bubbling Boilers, the entrance to the mine was recently re-boarded up once again until Thomas crashed through the blocked entrance and ran into the mine until he could not stop as he was covered with bubble liquid such as the runaways, brakes failure, bumps into mine trolleys, splashing into puddles of water, different directions and finally Thomas splashing into a waterfall, lands unbalanced and knocked off the rails.
As of the twenty-first series, the mine is operational again, as Philip is seen working at the mine as the lights go out leaving him in the dark in the episode, Dowager Hatt's Busy Day, and when Edward was trying to sleep there, only for dynamite blasts to bother his sleep.