In what has become a familiar refrain, lawmakers and local officials are calling for transportation funding to fill a budget hole that the state Department of Transportation has said would lead to service reductions on the Waterbury branch of Metro-North Railroad.

Unless the state finds new sources of revenue, the DOT will have to cut back weekend and off-peak service on the rail line — which runs seven trains a day between Waterbury and Bridgeport — and cancel plans to spend $8 million on new trains.

Lawmakers agree the branch should provide more frequent and reliable service to reach untapped demand, especially in light of a multimillion dollar upgrade that’s currently underway. The DOT is spending $70 million to signalize the railroad and add passing sidings, along with Positive Train Control, a system that can automatically slow or stop a train to prevent a collision.

But where to obtain the money to maintain existing service remains an open question.

Last Friday, U.S. Senator Christopher Murphy suggested a source other than new state taxes: federal funding.

During a rail forum at City Hall, Murphy, who is a member of the U.S. Senate Appropriations Transportation, Housing and Urban Development Committee, said the latest transportation bill provides $250 million more funding over the next six years than in the previous six.

Because Connecticut has three major interstates running through it, the state tends to receive a higher proportion of federal funding than others, he added.

Even if the money isn’t directly used to support rail, U.S. Rep. Elizabeth Esty, who is the vice ranking chair of the House Transportation and Infrastructure Committee, said it could offset the cost of other DOT expenses and free up money for the Waterbury branch.

Waterbury rail commuters often complain about the lack of service on the line, which has a single track and can only run one train in each direction at a time. That means when a rider misses their train, they have to wait 2 1/2 hours for the next departure.

With four passing sidings, the branch will be able to accommodate two-way traffic.

“If you put real, regular rail service on this line, ridership would explode,” Murphy said.

The branch has an average of 960 riders per weekday.

Rail service is integral to transit-oriented development initiatives, officials said.

In Naugatuck, a proposed transit-oriented development project on the former General DataComm property along Old Firehouse Road, locally known as Parcel B, relies on improved rail service on the Waterbury branch.

Murphy said the main reason the state can’t afford to support its transportation infrastructure is that its gas tax rate has remained the same since 1993, even though fuel consumption has declined. A federally mandated gas tax is possible, he said.

State lawmakers have also suggested electronic tolling and a mileage tax.