MBTA overseers gave a green light Monday to a $218 million contract for work to upgrade the train control signal systems on the Red Line and Orange Line, the final piece of work that T officials say is critical to meeting the system's service reliability goals.

The upgrades are intended to make it quicker and easier for the MBTA to make adjustments to track speed allowances and to repair the signal system when necessary. The new system will also work in tandem with the new Red Line and Orange Line cars the T plans to put into service over the next few years, allowing more trains to run more frequently.

"This is replacing 1970s-based analog signal systems along both the Orange Line and the Red Line. Beside the fact that we have components and relays that are obsolete now that we have to maintain in-house and overall reliability of the system is suffering, we are also limited in what we can do within the system in order to speed trains up, essentially," MBTA Deputy General Manager Jeff Gonneville said. He added, "We have to upgrade our signaling system at the T. Period."

The MBTA expects the signal upgrade work will be substantially complete on the Red Line by December 2021 and on the Orange Line by April 2022. Once paired with a fleet of brand new trains on both lines, the T expects to have trains run three minutes apart in the downtown core of the Red Line and four and a half minutes apart on the Orange Line.

"It gives us a modern system that allows us to actively manage the speeds when it comes to the signal process ... and it provides us with a new way to provide analytics around how our system is performing and things that are causing signal warnings," MBTA General Manager Luis Ramirez told reporters. "It's also exciting for us because once we get this in place and we also have the new vehicles with that technology in place, that's what gets us to the three-minute headways."

The signal upgrade project is part of the MBTA's $1.98 billion Red Line/Orange Line Improvement Program, which also includes purchasing 252 new Red Line cars and 152 new Orange Line cars, state-of-good-repair improvements, and other infrastructure projects.

Ramirez said Monday that the Orange Line should be done first, running all new trains on an upgraded signal system by 2023-2024, and that the Red Line will be in a position to reach the goal of three-minute headways between 2024 and 2025.

The MBTA said that, when completed, the Red Line/Orange Line Improvement Program will increase capacity on the Red Line, which currently serves an average of 244,000 riders on weekdays, by 50 percent and will increase capacity on the Orange Line, which currently accommodates 186,000 riders on an average weekday, by 40 percent.

MBTA commuters are accustomed to delays on the Red Line and Orange Line, as well as other lines, due to signal problems. The signal network essentially controls traffic along the MBTA's tracks through a series of circuits, switches and wiring. Only one train at a time is allowed in any given "signal block" and the signal system ensures that another train does not enter that block until the previous train has left it.

A faulty signal, though, can tell a train not to proceed down the track even when the signal block is unoccupied. Situations like that have led to numerous and frequent delays for MBTA commuters.

"This is something that, really, our customers are anxiously anticipating the day in which there will be fewer and fewer issues related to signals," MBTA Assistant General Manager for Capital Delivery Beth Larkin said Monday.

Because the new trains will have a shorter braking distance than the trains currently in use, the MBTA expects to be able to make some of its signal blocks shorter once the upgrades are complete, helping to increase capacity on the lines.

The T's Fiscal and Management Control Board on Monday approved a design-build contract with Barletta Heavy Division, Inc., for the Red Line and Orange Line signal upgrades worth up to $217,677,000. The T said it anticipates issuing a notice to proceed later this month and physical work will begin in about a year.

The current signal system will be replaced "along the wayside with next-generation digital audio frequency track service," Larkin said. The new system will be compatible with the older cars currently in service and the new fleet that will be coming into service starting early in 2019.

Gonneville said the new signal system will also have flexibility for the T to make changes as signal technology changes in the future. He said the system would allow the T to eventually consider things like automatic train operation, essentially self-driving trains.

"Even if 10 years from now there is a new solution, a better solution that will be the latest and greatest signal system that's out there, no matter what you have to have a good functioning signal system to build off of and this is that good functioning signal system," he said.

Once the work begins, Larkin said crews will work from north to south on each line. There will be 26 weekends with service diversions on the Red Line and 20 weekends of Orange Line service diversions to accommodate the work, she said.