The much-anticipated extended MBTA service begins tonight. While riders are excited about being able to stay out later, the increased service comes with a trade-off.

Keeping the nation’s oldest public transportation system running is not an easy task.

"As hard as we try we cannot keep up with the maintenance," said MBTA spokesperson Joe Pesaturo in an interview with WGBH News last year. "We’ve got the oldest subway in America. We’ve got limited resources. We have limited manpower."

And now the MBTA has limited time, as subway trains will be running until 3 a.m. on Saturday and Sunday mornings. This means the narrow window of time workers have to fix problems on the tracks just got narrower.

"If there are things that you know we could have gotten done between 1 in the morning and 3 in the morning are we going to be able to do that now? No." said MBTA Chief Operating Officer Sean McCarthy. "And that’s just a simple trade off that you have to make. You can’t have it all."

Signal, track and switch inspections are some of the regular Saturday and Sunday overnight duties that will need to be deferred due to the extended service, though McCarthy says the bulk of track maintenance work already takes place during the week. He says late-night service on the weekends is probably not going to have an impact to the T’s overall maintenance plan, which is expected to cost $290 million this year.

As for the future promise of 24-hour service? Don’t hold your breath.

"I would be skeptical at this point given our maintenance challenges," McCarthy said.

One major maintenance challenge: Work in the subway tunnels can only be done when the trains are not running. Unlike New York City’s subway system, which has multiple tracks, Boston’s subway only has one track in either direction — not enough room for both maintenance, and service.