Complex sentence: The SER's route could not compete with the Underground, which was more direct and reliable, but suburban traffic to Cannon Street remained popular, and the bridge was widened to in the late 1880s, allowing ten tracks with sidings.
Split into simpler sentences: The SER's route could not compete with the Underground, which was more direct and reliable. Suburban traffic to Cannon Street, however, remained popular, and the bridge was widened to in the late 1880s, allowing ten tracks with sidings.