Complex sentence: Some Atlanta streetcar lines were converted to "motor buses" (gasoline-powered buses) starting in 1925, but starting in 1937 some streetcar lines were converted instead to trolleybuses.
Split into simpler sentences: Some Atlanta streetcar lines were converted to buses starting in 1925. Those early conversions were to "motor buses" (gasoline-powered buses), but starting in 1937 some streetcar lines were converted instead to trolleybuses.