(AP) — One of the largest steam locomotives ever built is coming back to life.

The Union Pacific Railroad plans to restore an engine known as a Big Boy. It was a 6,300-horsepower monster designed to drag heavy freight trains over the Wyoming and Utah mountains.

Twenty-five Big Boys were built for the Union Pacific in the 1940s, but they were pushed aside by more efficient diesels by 1960.

The railroad plans to restore Big Boy No. 4014, which has been on display at the RailGiants Train Museum in California. It will be towed this month to Cheyenne, Wyo., where the work will be done at Union Pacific’s steam shop.

The project is expected to take three to five years. After that, the locomotive will be used to pull special excursion trains.