PORTLAND, Maine — Expansion of the International Marine Terminal on Commercial Street could begin this summer after the state purchased adjacent land and rail lines.

Maine Department of Transportation spokesman Ted Talbot confirmed Monday the department finalized the purchase of about 15 acres from Pan Am Railways.

Talbot did not have information on the purchase price, but said the acreage along West Commercial Street to Cassidy Point will allow the state to link the terminal by rail to the rest of the state and region.

But the state and neighboring landowner Phineas Sprague Jr. have not settled on a price for Sprague’s 18 acres, which are central to the terminal expansion. The state has already said it will take the property through eminent domain if necessary.

Sprague, a supporter of the terminal expansion, said he is still looking for a fair deal. “This is a normal process, a well-established process, and it is brutal,” he said.

The terminal is principally used by Icelandic shipper Eimskip for its container business.

Sprague owns Portland Yacht Services and had anticipated building a new boat yard on land where the state wants to build office and storage space, pier fill-ins and rail lines to accommodate rail traffic for containers. The $5 million expansion will be funded by $100 million of highway bonds approved by voters in November 2013.

Talbot said the rail line work could be sent out to bid by the end of the this month.

Sprague said he has an agreement with Pan Am to buy about five acres bisected by the railroad tracks that were sold to the state. He plans to open the boatyard there and hopes to have permits in place to begin construction in December.

“It is OK. It is process I understand and I am going to do the best I can,” Sprague said about the expansion and changes to his plans.

Patrick Arnold, director of operations and business development for the Maine Port Authority, which oversees the terminal, said Tuesday the container traffic continues to grow.

Projections for 5,000 containers to pass through the terminal in the first year proved accurate, and Arnold said last spring he was hoping for 6,000 in the second year of Eimskip business.

He anticipates gradual growth at an expanded terminal, as state and regional companies engaged in exports discover the rail-to-terminal link.

“If you really want it to work and you want to get a good return on investment on tax dollars, you have to give it time to work, and you have to put it into the right things,” Arnold said.