sentences:  They were built following successful tests of a prototype train-set called the "", derived from the MF 77, which tested new features such as interconnecting passages between cars to improve passenger distribution and special bogies to reduce friction caused by the sharp curves found in the Métro network.Carrying on the features of the BOA, a total of nine MF 88 train-sets were built, which currently operate on Line 7bis in a three-car formation.The high rate of wear-and-tear of the train-sets has made maintenance much more expensive than expected.