Complex sentence: To cope with the rise in traffic the line south of Harrow was quadrupled, in 1913 from Finchley Road to Kilburn, in 1915 to Wembley Park; the line from Finchley Road to Baker Street remained double track, causing a bottleneck.
Split into simpler sentences: To cope with the rise in traffic the line south of Harrow was quadrupled, in 1913 from Finchley Road to Kilburn, in 1915 to Wembley Park. However, the line from Finchley Road to Baker Street remained double track, causing a bottleneck.