Complex sentence: Operation continued through the 1960s with regular services every 7.5 min. However in the mid- 1970s the line was not modernized as pormissed no new rolling stock was acquired, so by 1975 only four railcars were in operational condition.
Split into simpler sentences: Operation continued through the 1960s with regular services every 7.5 min. However in the mid- 1970s the line was not modernized as promised, and no new rolling stock was acquired. Consequently by 1975 only four railcars were in operational condition.