Work has started on the Northwest Railway Museum’s latest addition, its Train Shed Exhibit Building.

The $4 million project is the second of several buildings planned for its campus on Stone Quarry Road.

More than one-third of the museum’s extensive collection of artifacts and vintage locomotives and railroad cars will move from downtown Snoqualmie to the 25,000-square-foot building. The museum now keeps these items in the open air, where the elements can wear them down.

“This is about sustainability,” said museum director Richard Anderson. “Wooden objects outside deteriorate rapidly.”

Bringing them into a climate-controlled environment will slow their deterioration to an “imperceptible” rate, he said. “Hopefully 100 years from now many of these artifacts will still be here.”

The “most vulnerable and representative pieces” of the museum’s 72 artifacts heavier than one ton will be moved to the new building.

The museum hopes exhibit building will enhance visitors’ experience a wider and more dynamic array of exhibits. The museum is hoping to restore a steam locomotive engine for children to see “what it was like to work on a locomotive,” Anderson said.

Currently, the museum has little room for exhibits and visitors are exposed to the weather, limiting most to good weather.

The museum expects to greatly add to the 85,000 visitors who come each year, Anderson said.

Visitors will enter the museum at the Snoqualmie Railroad Depot and take a short train ride to the Stone Quarry Road campus, which will have parking. The land for the campus was acquired in a land exchange with Snoqualmie and Meadowbrook Farm.

A 1912 Barney and Smith passenger car is being meticulously restored to carry visitors. They will “have a very similar experience to traveling up the Columbia River in 1920,” he said.

Despite the new building’s half-acre footprint, as many trees on the campus are being preserved as possible.

“We want the visitor to feel that the environment is bigger than the buildings,” Anderson said.

The museum has already secured $2.8 million of the building’s $3.5 million cost. Landscaping and additional expenses will raise the overall cost to $4 million.

The money has come from private individuals and foundations, federal and state grants, and revenue from museum events, such as Day Out with Thomas.

The construction site is being prepared for groundbreaking in early August. Total construction should take a couple years, according to Anderson.

“We still have a lot of work to do but we’ve come a long way,” he said.

The museum’s leaders conceived of the project seven years ago, but efforts could not begin until work was finished on the museum’s restoration building at its Stone Quarry Road campus.

“This represents a major step to realizing the vision of the people who founded the museum in 1957,” Anderson said.