Boston Mayor Marty Walsh called Wednesday for the MBTA to invest about $9 million in running Red Line and commuter rail trains more frequently, slamming the system — wracked with ongoing delays after a derailment last month — as "not currently a functional service" for much of the city.

His request, sent in a letter to Transportation Secretary Stephanie Pollack, came as MBTA officials forecast 10 to 20 minute delays on the Red Line continuing through at least Labor Day, as Gov. Charlie Baker pushes for more frequent shutdowns to speed up improvements across the rail system, and as lawmakers look for transportation financing solutions.

Despite ongoing repairs, Walsh wrote, "Red Line riders need better solutions today."

"I just signed a budget for the City that gives $90 million to the MBTA -- more than twice our own transportation budget," Walsh wrote. "As we know, the MBTA is not currently a functional service for many of the residents of Boston. Boston residents rely on the T to get to work, to take their children to school, to go to doctor appointments, grocery shopping, and live their lives -- my residents need the T."

Walsh asked the MBTA to run off-peak Red Line trains more frequently, similar to an increase in service on the Orange Line to accommodate the Encore Boston Harbor casino. He also suggested creating eight additional weekday trips on the Fairmount commuter rail line and on the so-called "South Shore Limited," which refers to expanded commuter rail service at four Red Line stations implemented following the derailment.

His proposals would collectively cost between $8.7 million and $9.4 million per year, about a third of the new revenue officials project from a set of fare increases that took effect July 1.

Those hikes drew protests on the first day and opposition from a range of elected officials, including Walsh, who wrote Wednesday that "riders continue to pay more for less."

"Taken together, these service improvements would create a positive, immediate benefit for riders in Boston," Walsh wrote. "We estimate that the total cost of these improvements is equivalent to a four month delay in fare increases and it's only fair that any additional revenue be invested directly back into improved service."

Transportation officials in the Baker administration have touted plans to spend record amounts on capital improvements. Asked for response to Walsh's letter Wednesday, MassDOT spokeswoman Jacquelyn Goddard said the department "share(s) Mayor Walsh’s interest in improving the public transportation system" and that Pollack and MBTA General Manager Steve Poftak "look forward to meeting with the Mayor to discuss how we are moving forward with plans" for infrastructure investment.

The mayor's demands come as frustration mounts with the MBTA and as Walsh himself grows more outspoken about the system. As recently as June 13 — two days after the Red Line derailment — Walsh had supported the fare hikes and, according to CommonWealth Magazine, called it "easy to Monday-morning quarterback" T operations after the incident.

But he changed tune a few days later, demanding a fare freeze and representation for Boston on the T's oversight board.

The MBTA said Tuesday that repairs to Red Line signal infrastructure are making progress and that the system will soon be able to run 28 trains at peak times as it did before the derailment.

Asked if commuters should still anticipate 10- to 20-minute delays, a T spokesman said only that riders should budget "extra time for their commutes."

Walsh's letter came on the same day that the Blue Line experienced significant disruptions during rush hour, apparently caused by "equipment failure" on a train that caused a power outage along much of the line.