EDMONDS — A proposed 7,800-foot trench to allow trains to pass through the city below grade would cost an estimated $250 million to $290 million, according to Tetra Tech, a Seattle consulting firm.

The trench would begin near the city’s off-leash dog park on the waterfront and would end near Caspers Street. It’s the latest suggestion to solve congestion around the city’s waterfront caused by passing trains — a problem the city has wrestled with for years.

The trench would help alleviate congestion around the waterfront by allowing traffic to pass above the tracks on bridges that would be built at Dayton and Main streets. The suggestion was made by two residents, Chuck and Katherine Gold, who developed a website, Edmonds Train Trench, to outline their proposal.

The city paid Tetra Tech $10,000 to do what Mayor Dave Earling described as a preliminary look at both the cost and the engineering challenges to be overcome if the trench is constructed.

Among the issues outlined in the report are excavating a trench that would be up to 30 feet deep in what is described as weak soil. The trench would need to be deepest from just north of Main Street to just south of Dayton Street, said Rick Schaefer, a senior program manager at Tetra Tech.

Other issues the project would face are proximity to the Puget Sound shoreline and to nearby steep slopes, shallow groundwater and the challenge of building the project while maintaining a rail line with frequent train trips, according to the report. The project would take about three years to complete, it says.

“It has to be technically achievable and it has to be judged as financially affordable,” Schaefer said.

There are a lot of complicated issues raised by the trench concept, said Patrick Doherty, the city’s economic and community services director. “Probably one of the biggest ones is the railroad needs to agree.”

Gus Melonas, a spokesman for the BNSF, said the railroad is reviewing the proposal and “will discuss details further with the city.”

Edmonds will ask the Legislature for $1.25 million to pay for a more in-depth look at how to solve the traffic issues in the city’s busy waterfront district, Doherty said. “If we’re able to get the money for an alternatives analysis, it would be at that point that we would decide” if the trench is feasible enough to include in the analysis, he said.

Traffic enters the city from the north and south to catch the Edmonds-Kingston ferry, used by by 3.8 million walk-on passengers and drivers each year. Ferry vehicle and pedestrian traffic must cross the train tracks when loading and disembarking.

Traffic delays also are caused by an increasing number of passing trains. About 40 trains go through the city each day, shutting down access to the city’s waterfront for about two hours a day, according to city officials.

On occasion, trains break down along the waterfront, blocking access to the area by both emergency vehicles and general traffic.

Earling has advocated for a study of transportation alternatives because train traffic is expected to increase significantly by 2030. If a projection of 100 trains a day is reached, it could shut down access to the waterfront and the state highway leading to the ferry for an estimated four-and-a-half hours in a 24-hour period, according to Earling. That would block access to the underwater dive park, the port and a senior center.

Sharon Salyer: 425-339-3486; salyer@heraldnet.com.