Negotiations are set to resume Monday between MTA management and Long Island Rail Road labor unions, according to a union source speaking on condition of anonymity.

The Metropolitan Transportation Authority and the unions are seeking to resolve a contract impasse that could affect thousands of commuters starting July 20, the strike deadline when workers have threatened to walk off the job. The LIRR is the nation's largest commuter rail system, with 300,000 daily trips.

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There were no signs of discussions Saturday at the Manhattan law offices of Proskauer Rose, which is hosting talks. On Thursday, when both sides last met, the unions presented a counteroffer that the MTA has said it is reviewing.

The area's delegation to Washington, D.C., has ruled out intervention by Congress, which has the power to impose a contract, order third-party arbitration and a range of other options.

The unions want the MTA to follow the findings of two mediation panels appointed by the White House. Both panels called for a six-year agreement with raises totaling 17 percent, employee health care contributions and no changes to pensions or to work rules.

The MTA wants those raises over seven years, as well as concessions from future workers, extending how long it takes to reach top pay and permanent pension contributions, instead of the current 10 years.