The M.T.A., which runs the system, said it would provide enhanced service on the M, G and 7 lines, as well as additional M14A buses and a new “Williamsburg Link” bus.

The disruption is expected to last about 15 months.

Why are disruptions necessary?

Workers need to repair the L-train tunnel that was damaged by Hurricane Sandy.

On other train lines, workers will upgrade the signal systems. Signals are sort of like traffic lights; the better the signals, the M.T.A. said, the greater number of trains it can run.

So, how do these upgrades get made?

Slowly.

Mr. Byford has a $40 billion plan that calls for installing modern signals on several lines. (State lawmakers have yet to give him the money to do it.) He wants to expand night and weekend closings in the coming years to accommodate the installations.

Some subway lines could see disruptions for more than two years.

Have any signal upgrades already been made?

So far, only the L and 7 trains have new signals, and those installations took longer than expected.

What lines are next?

Work is being done in phases. It has started on the Queens Boulevard line (the E, F, M and R trains) .