Complex sentence: As part of this, large-scale replacement of rolling stock was planned, then made up entirely of locomotive-hauled coaching stock, the most based on the British Rail Mark 2 and British Rail Mark 3.
Split into simpler sentences: As part of this, large-scale replacement of rolling stock was planned. At this time, this was made up entirely of locomotive hauled coaching stock, the majority of which was based on the British Rail Mark 2 and British Rail Mark 3.