More than six years later , the system is still crippled, sometimes by extensive repairs. Critical work on the tunnel that carries the L train between Manhattan and Brooklyn will begin tonight and last for at least 15 months.

To help commuters adjust to the plan — transit advocates called it the “L train slowdown” — there will be changes to how you can get around, including vehicle restrictions on the heart of 14th Street in Manhattan.

Here’s what you need to know:

L train service

Starting tonight at 8, service will be reduced on weeknights and weekends. After 10 p.m. and on weekends, trains will run every 20 minutes.

The Metropolitan Transportation Authority will run extra G, M and 7 trains, as well as buses, including two special Williamsburg Link bus routes.

Bike lane upgrades will be made permanent on Grand Street in Brooklyn, and on 12th and 13th Streets in Manhattan.

Driving on 14th Street

Starting in June, private vehicles will not be allowed to travel on 14th Street between Third and Ninth Avenues, which runs parallel to the L train.

Buses, delivery trucks and cars making pickups and drop-offs will be permitted curbside access on 14th Street, but will be required to make a right turn off the street at the first opportunity.

This portion of 14th Street will have four lanes of traffic : two in each direction, with the center lanes dedicated to buses.

Enforcement of the new rules will start about 60 days after they begin, the city said.

The back story

To repair the Canarsie Tube, which links Manhattan and Brooklyn, transit officials had to choose: stop all traffic and make repairs full time, or reduce traffic and work part time.

The M.T.A. chose the first option, but then Governor Cuomo abruptly intervened. He decided the state should instead rely on new technology (which critics have called untested) to complete the job during a reduction in service on nights and weekends.