Complex sentence: The modification was the inclusion of an additional ("inside ") cylinder between the frames, and the inclusion of a driving axle for the middle driving wheel connected to the"inside" cylinder assembly by a connecting rod.
Split into simpler sentences: The modification was the inclusion of an additional ("inside ") cylinder between the frames, and a driving axle was fitted to the middle driving wheel. The axle was connected to the"inside" cylinder assembly by a connecting rod.