The two land masses that were once Tom Sawyer Island are now piles of dirt in the middle of Magic Kingdom. The area is being turned into Piston Peak National Park, a Cars expansion to Frontierland, while a new Villains Land is underway closer to Big Thunder Mountain Railroad.
Magic Kingdom Construction

Aerial photographer @bioreconstruct on X shared these photos of Magic Kingdom, revealing that all the trees have been removed from Tom Sawyer Island (a few remained in the last aerial update).
Piston Peak is expected to be in the circular area sandwiched between Liberty Square and Frontierland, while Villains Land will take up the space next to and beyond Big Thunder Mountain. In a backstage area that used to be full of trees, crews are placing sand on the ground.

There appears to be the remnants of a rocky formation in the middle of the former Tom Sawyer Island. The smaller island held most of the Tom Sawyer Island offerings, while the larger island had Fort Langhorn.
With the expansion, guests will be able to walk directly from The Haunted Mansion to Big Thunder Mountain Railroad.

Due to construction, the Walt Disney World Railroad is running in “shuttle mode” back and forth between Storybook Circus and Main Street. Its backstage track appears to have been completely removed between Big Thunder Mountain and Fantasyland.

Big Thunder Mountain Railroad is currently closed for a major refurbishment but will reopen next year. We saw refurbished trains testing in November.
Disney has not announced opening timeframes for the Cars expansion and Villains Land.
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