CHICAGO (CBS) — The CTA is marking another milestone in the modernization of its rapid transit fleet.

While many riders may not like the 5000-series ‘L’ cars now under construction, they have allowed CTA to retire its two oldest series of rapid transit cars.

Fifteen months after retiring the last of the 2200-series ‘L’ cars built beginning in 1969, during the first Nixon presidency, CTA has retired the next-oldest group of ‘L’ cars, the 2400-series cars delivered by Boeing beginning in the fall of 1976 and christened by the first Mayor Daley two months before his death.

The 2400-series cars were seen all over its rail system until this past year. Their last assignment, oddly enough, was the Orange Line — a place they had never run till recently. The last were removed from service after the Friday morning rush hour. A couple dozen of the cars, with red-and-white stripes on the ends and sides, will remain indefinitely for maintenance purposes.

The last of the controversial 5000-series ‘L’ cars should be delivered next year. More than 550 of the planned 714 cars are now in service. CTA has released specifications for its next group of rapid transit cars, which would allow CTA to retire all cars older than the 5000 series by 2022.