Complex sentence: The first class of modern tramcars (15 tramcars) were equipped with trolley poles as the PCC were still in use, there were subsequently replaced by pantographs as the PCC were withdrawn and the second batch of modern cars was introduced.
Split into simpler sentences: The first class of modern tramcars (15) were equipped with trolley poles since the PCC streetcars were still in use. These were subsequently replaced by pantographs as the PCC streetcars were withdrawn and the second group of modern cars was introduced.