PORTLAND – The start of Amtrak service between Portland and Boston has been delayed again because of a lingering dispute linked to the passenger train’s speed and safety.

The Northern New England Rail Passenger Authority, which is overseeing the project, has scrapped its tentative starting date of May 1, and is no longer projecting when the service will start.

The latest delay could be for several more months, and possibly longer. If the project is delayed until after the summer tourist season, officials should think about delaying passenger service until next spring, said Jeff Monroe, Portland’s transportation director.

At issue is whether passenger trains will be allowed to run at 59 mph or 79 mph. A federal board ruled two years ago that the trains could run safely at the higher speed if the track was upgraded. But the rail authority and Guilford Rail System, which owns 78 miles of track between Portland and Plaistow, N.H., are at odds over how to make sure that the track meets federal standards.

Guilford executive David Fink said Tuesday there is no acceptable testing method available, and that the only alternative is a costly upgrade of the rail bed. He said Guilford doesn’t believe that the crushed granite being installed under the tracks now is deep enough to meet requirements for rail rigidity.

The work on the track is nearly complete. Guilford, which is being paid tens of millions of federal dollars to do the work, expects to finish in about two months.

The rail authority will seek a ruling on the latest dispute from the federal Surface Transportation Board, which typically takes three to six months to make a ruling.

Supporters of the train service were angered at news of another delay.

Phil Harriman, a former legislator from Yarmouth who was involved in rail issues, said the upgraded track exceeds the safety standards of others that Amtrak uses. He said he suspects that Guilford is trying to find ways to prevent passenger service from living up to the public’s expectations.

“Frankly, this is a running joke, that this [train service] is all a mirage and this isn’t going to happen,” he said. “The more Guilford deploys these tactics, I’m sure it fuels people’s skepticism that it won’t happen.”

The line will have stops in Portland, Saco, Wells and Old Orchard Beach. In Old Orchard Beach, where officials have been working on plans for a station since 1993, officials were angry but not surprised.

“The taxpayers have invested a lot of money into Guilford,” said Town Planner Tad Redway, “and until this point Guilford is the sole beneficiary.”

Fink said Guilford is thinking only about the safety of the passenger service and still wants the project completed.

Another unresolved issue is Guilford’s refusal to let any platforms be built at train stations along the route until the state buys insurance to cover any environmental cleanup costs.