The Monday morning rush hour was upended on New York City’s subway after multiple train lines faced rolling delays — some of them “severe” — according to the MTA.

The transit agency said at one point that service on a dozen lines was disrupted due to either infrastructure issues or medical emergencies.

As of around 8:15 a.m., service was severely delayed on the 2, 4, 5 and B trains, the MTA’s website showed. There were delays in both directions on the first three lines because of an earlier signal malfunction near the Nevins Street station in Brooklyn. Downtown B trains were backed up after first responders responded to someone needing medical help at the Bedford Park Boulevard station in the Bronx.

The MTA also reported significant delays on the 3, 7, M, J and Z trains. The latter three trains were dealing with signal problems on the Williamsburg Bridge, according to the transit agency. The delays on the 7 train were ascribed to a police response after passengers were being disruptive at the 40th Street-Lowery Street station in Queens.

Earlier in the morning, Manhattan-bound E and downtown F trains were delayed as emergency medical workers responded to someone needing medical help at the Forest Hills-71st Avenue station in Queens.

Modernizing signal systems is part of the MTA’s latest five-year construction plan. The $5.4 billion investment will add a more efficient and upgraded signal system to various lines, transit officials have said.