Complex sentence: These three cylinder Pacifics, although capable of hauling heavy loads at high speed, had in practice proved to be relatively maintenance-intensive, particularly with regard to the servicing of the valve gear and motion for their third (inside cylinder).
Split into simpler sentences: These three cylinder Pacifics, although capable of hauling heavy loads at high speed, were in practice relatively maintenance-intensive. Key issues related to the servicing of the valve gear and motion for their third (inside cylinder).