Complex sentence: The Class 158 and Class 159 diesel multiple units, although superficially similar, are not based on the Mark 4 bodyshell: the Mark 4 is derived from the Class 156.
Split into simpler sentences: The Class 158 and Class 159 diesel multiple units, although superficially similar, are not based on the Mark 4 bodyshell. In fact, the Mark 4 is derived from the Class 156.