PORTLAND – Amtrak’s Downeaster will use buses for an additional daily round trip between Portland and Boston while it awaits completion of $6 million in track upgrades that will allow the train to make the extra run next year, officials said Thursday.

The Northern New England Passenger Rail Authority has partnered with two bus operators to allow the Downeaster to press forward with a fifth round trip beginning on Oct. 30. Under the arrangement, the last run of the night will be operated by a bus, not a train, until the track upgrades are completed.

Eventually, all five round trips would utilize Amtrak’s Downeaster once the track work is completed, which is expected by next fall, said Patricia Douglas, the rail authority’s executive director.

The fast-growing Downeaster wrapped up the fiscal year with 329,265 passengers – an increase of 31 percent over the previous year. Revenues grew to $4.35 million, surpassing the $3.3 million during the previous fiscal year.

“Although we’ve been extremely successful this year, we know that the existing four round-trip schedule limits the number of riders we can attract. The goal is to operate more trains during the times when most people travel,” she said.

Under the arrangement, all Downeaster trains will leave Portland earlier, with morning departures of 5:50 a.m. and 8 a.m. instead of 6 a.m. and 8:50 a.m. on weekdays. This will help to get commuters to Boston earlier, Douglas said.

The track work that’s under way involves new sidings to allow freight and passenger trains to move around each other, Douglas said. “To increase frequency, you need more places for trains to pass each other,” she said.

The work is being done by the railroad’s owner, Pan Am Railways, formerly known as Guilford, which owns the rail between Portland and Plaistow, N.H. It’s being funded by federal money allocated by Maine and New Hampshire, Douglas said.

Meanwhile, the rail authority and the Massachusetts Bay Transportation Authority worked together to obtain federal funds to complete a signal project in Lawrence, Mass., that will ultimately provide better access to Boston’s North Station.

The Downeaster went into operation on Dec. 15, 2001, with four daily trips in each direction between Portland and Boston’s North Station.

Stops include Old Orchard Beach, Saco and Wells in Maine; Dover, Durham and Exeter in New Hampshire; and Haverhill and Woburn in Massachusetts.