The MBTA is deep into a multibillion- dollar campaign of end-to-end fixes, from patching leaks and fixing crumbling stairs, to improving signs and cleaning stations, to adding sophisticated technology such as electronic fares and new signals that allow for faster subway service.

But the agency’s track record with one component of its infrastructure — elevators — has been marked by delays and expensive workarounds, leaving riders who depend on them to endure months of longer, inconvenient commutes, or arduous trips up often poorly maintained stairs.

One elevator at Harvard Square station was supposed to be out for a year, with a new one in place by spring 2019; the T now doesn’t expect it to return until the end of this year. An elevator at Central Square station has been out of service for two years, as the T encountered multiple complications with other equipment nearby; for some of that work, contractors were limited to just three hours a night when the power could be cut off.

The delay was more modest at Andrew Station, where the replacement of three elevators is a few weeks behind schedule and is now expected to be completed soon. The outage has lasted 13 months.

“I know elevators are not an easy thing to replace, but it shouldn’t have taken this long,’’ Will “Nick’’ Crow, who has cerebral palsy and uses a wheelchair, said as he waited outside Broadway Station for a shuttle to take him to Andrew.

The shuttle service between Andrew and Broadway, and a separate one between Central and Kendall, runs barely a mile in each direction, and each one attracts around 10 passengers a day. So far, the T said it has spent $300,000 on the Andrew shuttle; the Central-Kendall one runs $36,000 a month.

The shuttles are not an ideal solution for some passengers. Daniel Hensley, 66, who walks with a cane after a car accident, visits friends and goes to a clinic in Cambridge, coming through Central Station.

“I have to gingerly make my way up the stairs,’’ said Hensley. “I’ve used the shuttle service before, but I think it’s not enough, because if you’re in a hurry, it’s no good.’’

Valerie Fletcher, executive director of the Institute for Human Centered Design, called the elevator delays “unconscionable.’’

“I can’t fathom that anybody would think that’s a normal condition or acceptable,’’ said Fletcher, who helped write the MBTA Guide to Access in 1990. “If you just hang out at any of our stations in the morning, there’s a large number of people who have difficulty walking. They’re not obvious people who suffer when there’s no elevator, but they’re absolutely part of a population that is deeply disadvantaged.’’

But these kind of dislocations may be routine in the coming years. The T has ambitious plans to fix and replace elevators all across the system. On the Red Line alone, the agency identified $137 million in elevator upgrades, and tens of millions of dollars more for elevators elsewhere.

The T points out that all the elevator work is itself a sign of improvement. In part from a 2006 settlement with an accessibility organization, the T has conducted a “a concerted and robust effort’’ to stay on top of elevator maintenance and replacement, a spokeswoman said, instead of just waiting for them to break as once was the practice.

Since the settlement, spokeswoman Lisa Battiston said, the agency has 50 new elevators, 10 under construction, and 60 more in design. The overall reliability of the elevators has improved, too, she said, to the point where the entire fleet is consistently available more than 99 percent of the time the T is in operation.

T records indicate that currently four stations have elevators out for an extended period because of construction. Elevators at other stations are off-line for brief periods, as short as one day, for maintenance.

Battiston said the average time to completely replace an elevator ranges from six to 12 months, depending on the size of the job and conditions at the stations, such as access to the work area.

“That said,’’ Battiston added, “unforeseen existing conditions that are encountered during construction may cause projects to take much longer than anticipated to address.’’

At Andrew, the replacement work was originally expected to be finished in August, but Battiston said water and structural issues slowed the work, pushing the outage to more than a year.

But Gerald Clark, a principal consultant at the Escalator Elevator Consulting Group who was previously in charge of elevators and escalators for the Bay Area Rapid Transit in California, questioned the lengthy repair schedule, saying it should take about three months to fix or replace an elevator.

“The elevator work itself shouldn’t take a year,’’ Clark said. “Transit agencies are not real good with doing forward capital replacement work on existing stations — they don’t think ahead with a 20-year plan. They’re real slow, but there are a lot of factors, including bureaucratic barriers.’’

The T has been steadily trying to accelerate its efforts to fix and modernize the system, a task made more urgent by the June derailment of a Red Line car that reduced service levels for much of the summer. The Baker administration has since begun to shut core sections of the Red, Orange, and Green lines on weekends to speed up necessary repairs.

Baker often touts the T’s current five-year capital plan of $8 billion as the largest in the agency’s history, and separately the T last year developed a new plan to track the condition of its holdings and prioritize projects. The list of “investment priorities’’ runs a staggering 27 pages, and includes several programs to bring elevators up to current standards for accessibility.

The Harvard and Central elevator work is combined into a single $7.1 million contract. The same contractor also has the Andrew station work, which the agency priced at $3.5 million.

At Andrew, Gregory Wallace is looking forward to the return of the three elevators. During the outage, the South Boston resident, who has knee problems, lugs his shopping cart full of groceries up the station stairs several times a week, and there isn’t an escalator up from the northbound platform.

“I bring stuff to my mom, and getting off here is easier than at Broadway — rush-hour traffic over there is crazy,’’ Wallace said. “They could have built four battleships in the time that they’ve been working on that elevator.’’

Diamond Naga Siu can be reached at diamondnaga.siu @globe.com. Follow her on Twitter @diamondnagasiu