Complex sentence: The high rate of wear-and-tear of the train-sets has made maintenance much more expensive than expected and it was speculated that they were likely to be replaced by spare MF 67s, which in turn were replaced by the MF 2000.
Split into simpler sentences: The high rate of wear-and-tear of the train-sets has made maintenance much more expensive than expected. It was speculated that they were likely to be replaced by spare MF 67s, which in turn were replaced by the MF 2000.