PORTLAND – The Northern New England Rail Authority is proposing a new fare system for the Downeaster that is modeled on that of airlines, with higher prices for the most popular runs and lower prices for off-peak ones.

Under the proposal, fares for the Portland-to-Boston line would increase more than 4 percent overall.

The fare plan would be designed to change according to advanced bookings, so there is no list of how each run will be affected.

Fares would also change with the seasons, rising in the summer on many trains but then dropping after Labor Day.

“It’s really a supply and demand situation,” said Barbara Whitten, a member of the rail authority board of directors.

The biggest jump on a Portland-to-Boston ticket would be $2, from $21 to $23.

For passengers departing from New Hampshire and Haverhill, Mass., fares would increase from 7 to 15 percent. Commuters traveling within Maine would enjoy lower ticket prices.

Maine put $400,000 in taxpayer money and another $1.6 million of federal grant money it obtained into the service this year, but New Hampshire and Massachusetts do not subsidize the Downeaster.

The federal grant money will expire after three years, and the state will have to find a way to make up the difference.

Rail authority officials said they want to show Maine lawmakers that they are doing what they can to minimize the state’s subsidy.

The price of commuter passes and 10-trip tickets would not be changed.

The round-trip fare would still be available for $35, but passengers would have to travel during off-peak hours. The ticket now requires purchasers to travel on the same day, but the proposal would allow travel over multiple days.

The proposed changes are part of a strategy to bring in the most revenue while filling the trains with the greatest number of passengers.

“This is not done out of the desire to raise money for the heck of it,” said John Englert, NNEPRA’s executive director. “But a matter of trying to eke as much from our existing infrastructure as we possibly can. It is simply smart business. That’s all.”

The rail authority wants to entice more passengers onto some weekday trains and get more regular riders.

The 10-day pass, which is valid for 10 one-way trips within a 45-day period, would give holders a 20 percent discount.

Englert said the Downeaster’s costs are rising because of scheduled union wage increases and a sharp rise in insurance costs.

It’s estimated that the Downeaster’s costs will total $6.9 million this year. Revenues were $4.7 million.

The cost is projected to rise to $7.1 million next year.

NNEPRA’s board plans to vote on the plan Jan. 10. The public can comment by going to the Downeaster’s Web site: www.thedowneaster.com