ONE HUNDRED AND SIX days after a Red Line train derailed and smashed into signal equipment, the MBTA announced Wednesday afternoon that it has finally restored service to normal on the transit authority’s busiest subway line.

The crash forced the T to shift to a manual signal system, where workers located at each station using flags and radios communicated when it was safe for a train to move on to the next station. The manual process slowed operations and caused extensive delays.

Recovery from the crash was a multi-step process with workers reasserting automatic signal control over stretches of track piece by piece. The crash occurred where the southern legs of the Red Line – from Braintree and Dorchester – merge together at JFK/UMass. The last stretch needing repair was on the Braintree section of the track just north of the merge. An MBTA spokesman said the total cost of the Red Line repair work in terms of wages, materials, and services was $5.4 million.

“While I’m pleased Red Line service has been restored, this event underscores the level of urgency we need to continue to build a better T,” said MBTA General Manager Steve Poftak in a statement. “I want to express my deepest gratitude to our customers for their patience while restoration work took place. I also want to thank our entire workforce and contractors for their efforts to work seven days a week to restore the system to normal.”

Some other aspects of Red Line service were back to normal before Wednesday’s announcement. As of last week, according to the T, the frequency of service in the downtown area was back to about one train every four minutes during rush hour. Right after the crash, trains only ran about once every 10 minutes, and signaling along the affected areas was done manually.

With the signal systems restored, countdown clocks in Red Line stations are back to working order. The derailment, and its aftermath, appear to have taken a toll on rider satisfaction and also the number of people using the Red Line. T officials on Wednesday said the latest ridership numbers were not immediately available.

Immediately after the crash, Poftak said the T planned to replace the signal systems near JFK/UMass next year, and MBTA spokesman Joe Pesaturo on Wednesday said that is still the goal, but T officials are in talks with the contractor about the final schedule.

Pesaturo said a new signal shed was built under Interstate 93, and the future project to replace all the signals in that area will entail building a second bungalow under the expressway, which runs alongside the Red Line tracks.

Gov. Charlie Baker credited a nighttime visit to the crash site with convincing him to accelerate repair work throughout the system, even if that means temporarily shutting down service on nights and weekends.