BLOSSBURG, Mont. (AP) A train tunnel beneath the Continental Divide is undergoing its first major overhaul since it was built more than 120 years ago.

The 3,896-ft. (1,190 m) Mullan Tunnel is the longest rail tunnel in Montana. At less than 13 ft. (4 m) wide, it also is one of the narrowest — some freight cars clear the tunnel with less than 3 in. (7.6 cm) to spare on either side.

The tunnel is located on a vital rail line between Helena and Missoula. The $18 million renovation by regional railroad Montana Rail Link will make the tunnel 5 ft. (1.5 m) taller and 3 ft. (.9 m) wider.

Montana Rail Link officials say a bigger tunnel will better accommodate today’s larger freight trains while helping to prevent locomotives from overheating.

The overhaul by L.R.L. of Tillamook, Ore., presents a structural and logistical challenge. The tunnel originally was shored up with timbers, padded with concrete walls, and capped with an arch of red brick that was baked and stamped on site.

Crews will remove the material down to bare stone.

“We’ve got some weathered material, which can be a challenge at times,” said Ray Jordan, a tunnel inspector with Jacobs and Associates. “If we have a pocket, I want to make sure we can continue to tunnel in a safe manner to pass trains so that 100 years down the road it’s still going to operate.”

Crews have begun working to remove a large chunk of the mountain above the tunnel’s west side. They plan to set back the tunnel entrance 400 ft. (121 m), reducing the tunnel length to 3,496 ft. (1,070 m).

The blackened walls and lack of light make for tedious, dirty work. Crews wear respirators, hard hats and boots and work under the glow of construction lights.

“It’s black, dark and dirty,” said tunnel worker Brian Brown. “There’s old soot on the walls and stuff. You get worried from time to time, but it’s not too bad.”

As many as 25 employees are working on the project including blasters, excavators, geologists and railroad officials.

Work on the original tunnel began Dec. 14, 1881. Crews cut down trees and used the wood as they went. In 1883, groundwater burst into the tunnel — carrying away support beams, depositing debris and slowing progress — but locomotives began chugging through later that year.

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