Query: What mode of public transportation in New Haven phased out the trolley service in the 1920's and 1930's?
Context: Horse-drawn steetcars began operating in New Haven in the 1860s, and by the mid-1890s all the lines had become electric. In the 1920s and 1930s, some of the trolley lines began to be replaced by bus lines, with the last trolley route converted to bus in 1948. The City of New Haven is in the very early stages of considering the restoration of streetcar (light-rail) service, which has been absent since the postwar period.