Complex sentence: A single line was converted to dual gauge in 1868 to allow P&T trains to reach, but it wasn't until 1872 that full through running between the two routes was possible with the conversion of the main line to standard gauge.
Split into simpler sentences: A single line was converted to dual gauge in 1868 to allow P&T trains to reach. Full-through-running trains between the two routes was made possible in 1872 with the conversion of the main line to standard gauge.