U.S. Rep. Seth Moulton wants the state to approve construction of a new rail tunnel below downtown Boston, but the mega-project is turning into a competition with another big rail expansion.

Moulton made his pitch to business and civic leaders for the North-South Rail Link, a tunnel that would connect North Station to South Station. That would allow trains to run all the way through Boston, instead of terminating on either side of downtown.

The North Shore congressman says the idea is much better than the one Gov. Charlie Baker prefers—which is to expand South Station to accommodate more commuter rail trains.

"I think South Station expansion would be a historic mistake for the state," Moulton said. "Because we really should be using the money set aside for South Station expansion for the rail link. It makes no sense to do both projects because the rail link solves the capacity problem at South Station."

The Massachusetts Department of Transportation is analyzing the feasibility of the tunnel, but for now, still plans to pursue a bigger South Station.