A/B skip-stop service debuted on the Lake Street Elevated in 1948, and the service proved effective as travel times were cut by a third. By the 1950s, the service was used throughout the system. All lines used the A/B skip-stop service between the 1950s and the 1990s with the exception of the Evanston and Skokie lines, which were suburban-only lines and did not justify skip-stop service. On the lines with branches, skip-stop service sent all "A" trains to one branch and "B" trains to another branch. On what became the Blue Line, "A" trains were routed on the Congress branch while "B" trains were sent to the Douglas branch. On the North-South Line, "A" trains went to the Englewood branch and "B" trains went to the Jackson Park branch. In both cases, individual stops were not skipped beyond the points where those branches diverged. As time went by, the time periods which employed skip-stop service gradually decreased, as the waits at "A" and "B" stations became increasingly longer during non-peak service.