Complex sentence: One wave began with the streetcar lines that made many parts of Newton accessible for commuters in the late nineteenth century, the next wave came in the 1920s when automobiles became affordable to a growing upper middle class.
Split into simpler sentences: One wave began with the streetcar lines that made many parts of Newton accessible for commuters in the late nineteenth century. The next wave came in the 1920s when automobiles became affordable to a growing upper middle class.