Complex sentence: In 1936, they bought 13 transit companies in Illinois, Oklahoma and Michigan, then in 1937, they replaced streetcars in Butte, Montana and made purchases in Mississippi and Texas.
Split into simpler sentences: In 1936, they bought 13 transit companies in Illinois, Oklahoma and Michigan. Then, in 1937, they replaced streetcars in Butte, Montana and made purchases in Mississippi and Texas.