PORTLAND – The midmorning run on Amtrak’s Downeaster from Portland to Boston has proved more popular than expected, prompting local rail officials to ask Amtrak to add another passenger car.

The 8:45 a.m. train usually rolls into North Station with people standing, said Michael Murray, executive director of the Northern New England Passenger Rail Authority.

Wednesday’s train had its normal complement of three passenger cars, with 60 seats each, and the food service car, with seating for 40 more. It had 60 people standing by the end of the trip, Murray said.

Amtrak’s decision on whether to add a passenger car is expected within a week, he said.

Amtrak should have an extra car available, Murray said, because the rail system has three complete trains for use on the Downeaster, although it usually needs only two. A locomotive and passenger car are stored in Portland as spares.

More than 4,300 people took the train during the first five days of operations, which began Dec. 15. That works out to an average of 860 passengers a day.

Amtrak estimates that 320,000 people will ride the Downeaster each year, an average of 877 a day, but ridership is expected to be higher in the summer than the winter.

Murray acknowledged that ridership the first few days might have been inflated by the novelty of train service.

The number of passengers riding the Downeaster is expected to decline after Jan. 1, until the next school vacation in February, he said. Then again, ridership could increase if there is snow, because people going to Boston may decide to take the train instead of driving.

“It’s a learning process,” Murray said. “I have to see what trends develop here. It would be easy to fall into that foolish trap and say, ‘We’ve had tremendously robust ridership in the last few weeks,’ but what’s going to happen after the first of the year? I have to be a realist here.”

The rail authority is soliciting companies to advertise on the cars, to be exclusive suppliers of food and beverages and to sponsor trip packages. Under discussion with the Boston Bruins hockey team is a proposed package that combines train tickets with tickets for a game.

Murray also said the authority hopes to learn in January whether it can increase the speed of the trains from the current 60 mph to 79 mph, a move that would cut 15 minutes off the two-hour trip.