Major Bronx 4/5 train service changes are in effect until late February. Here’s what you need to know.

Late-night and weekend service changes on Bronx 4 and 5 trains that took effect on Monday and will run through late February, the MTA announced.

The MTA is making the adjustments so its crews can replace 37-year-old switches that move trains safely between the Bronx and Manhattan, according to the agency. The work is aimed at reducing dealys and improving service for roughly 1.1 million straphangers.

Once installed, the new switches are expected to last for 20 years, the MTA said on Tuesday.

As of this past Monday, 4 trains are operating in two sections, between Woodlawn and 149 St–Grand Concourse in the Bronx, and the Bronx’s 3rd Ave–138 St 6 train station and New Lots Av in Brooklyn. They are making all local stops in Manhattan.

The changes are in effect weeknights between 11:45 p.m. and 5 a.m. until Feb. 20.

The same switch-up will take place on five consecutive weekends between Jan. 23 and Feb. 23. They will begin around 9:45 p.m. on Fridays and run until 5 a.m. on Mondays — except for Presidents’ Day weekend, where the changes will last until 5 a.m. on Tuesday instead.

On those weekends, 5 trains will function as a shuttle between Eastchester–Dyre Av and E 180 St. in the Bronx.

Although most 4 and 5 trains will run regular weekday service during the same period, some 4 trains will be rerouted to 3 Av–138 St and some 6 trains will run on the 2 line between 149 St–Grand Concourse and Nevins St during rush hour. Those changes will occur between Monday, Jan. 26, and Friday, Feb. 20.

The MTA will run free shuttle buses between 149 St–Grand Concourse and 3 Av–138 St over the weekends when its crews are conducting the work.

The agency is also urging riders to take the 2 or D train or Metro North Railroad’s Hudson or Harlem lines in the Bronx — where fares will be cross-honored — as alternatives.