As of 1 April 2012, two 6-car sets were in service, formed as follows with three motored "M" cars per set.

The latest M car uses the chassis and suspension from the larger M3 plus a modified, widened 1-series body.

The M car is fitted with two pantographs.

The fleet could be seen running in any arrangement, from one car (using a double-ended M car), up to eight cars.

The remaining M cars were renumbered from 273M up to 300M, however the new numbering does not reflect the age of each car.

After these upgrades were made, the non-upgraded M cars were no longer allowed to lead a train in revenue service, effectively making them B units.

But an M car should always combine efficiency with emotion.

The design style is similar to all recent M cars.

The two "M" cars are each fitted with two scissors-type pantographs.

Accordingly, "an M car has to be responsive and fundamentally keen on turning as well as accelerating.