They are at their worst on narrow city streets. Cities were built long before the invention of the automobile. Their higher density use of land makes them very unfriendly to large numbers of automobiles. Light Rail Transit would enable large numbers of people to come into the city without bringing automobile traffic with them. Land developers see a rail transit line as an economic incentive and are encouraged to build next to or very near them.
Rail lines encourage more compact development along corridors.
Think of sprawl as a splash of water on a sidewalk. Think of corridor development as fingers on a glove. That's the difference. Traffic jams killed the cities. To survive in the 21 st Century, cities must have forms of transportation that favor their type of development.