A/B Skip-Stop Express Service


When the CTA took over "L" service on October 1, 1947, the system was running an extremely complex and cumbersome system of express and local schedules. The trouble was, the rolling stock was aging (most of it still wood cars from the turn of the century) and slow, most lines were only two tracks (a few were sections had a third express track; only the North Side Main Line had four tracks for separate express and local service in both directions) with train getting in each other's way, and the services were hard for riders to comprehend. Ridership was plummeting fast.

The CTA decided that something had to be done. New all-steel, all-electric PCC rolling stock was ordered, but it would be years before all the wood cars could be replaced and besides, that would likely not be enough. After several years of studying the routes, the CTA devised a clever way of running express service on its two-track lines. Dubbed the "A/B skip-stop" plan, the service is best explained in this CTA informational bulletin:


Why the "A" and "B" plan was developed:

CTA acquired the surface transit system and the elevated-subway system from two closely competitive companies. After the consolidation, various means were explored of coordinating the services and thereby diverting long-haul traffic from buses to rapid transit.

Most of the rapid transit lines in Chicago are double track, without paralleling express tracks. Schedule speed is limited by the number and duration of station stops and trains ahead as well as by the performance capability of the car equipment over the available alignment and profile.

Performance of cars is improved with each successive design and much effort has been made to increase the average distance between station stops. To accomplish this, some lightly used 'L' stations were removed and a skip-stop "A" and "B" service was established.

What "A" and "B" service is:

In the "A" and "B' plan of express service successive trains are alternately "A" trains and "B" trains. Less important stations on the route are designated alternately from the end of a route as "A" stations or as "B" stations. More important stations are designated as "all-stop" stations. "A" stations are served by "A" trains, "B" stations are served by "B" trains and "all-stop" stations are served by "A" and "B" trains. The "A" trains skip one group of stops while the "B" trains skip an alternate group. All are expresses and there are no locals.

Principal advantages gained:

The "A" and "B" plan of operation...

  • increases train speeds, thus making the service more attractive for passengers. This benefit is extended to passengers on all portions of the routes
  • provides service frequency somewhat proportioned to riding demand by giving the more heavily patronized local-stop stations more frequent train service than relatively lightly used "A" and "B" stations,
  • permits fine adjustment of headways without the handicap of severely unbalancing loading between trains as is the case with more conventional local-and-express service, and
  • reduces car requirements, manpower requirements and operating costs compared to all-local or local-express plans.


Thus, the CTA devised a way to run express trains on lines where no passing tracks were available. Interestingly, on two of the lines where a third center track was available for express service -- the Lake Street and South Side Main Line -- the CTA chose to abandon them. Under the A/B system, "A" and "B" trains never got fouled behind one another (at least theoretically) because by having them skip alternate stops each train never caught up with its leader and the headway at the "AB" or All-Stop stations was practically consistent, allowing trains to get more or less equal loads.

Symbols of A/B Skip-Stop Service: The symbols above were used as part of the A/B system. All four were used on maps. The top two were used on station column signs from the 1950s to 1969 and on destination signs.

The first route the CTA implemented A/B service on was the Lake Street Line, chosen because of the slow equipment, close station spacing, and number of parallel, duplicative surface lines. Implemented on April 5, 1948 and including the closure of 10 little used stations, including the Market Street Stub, and discontinuing the short-turning of trains mid-line at Hamlin and Austin, the A/B system cut running times on the Lake Street Line by a third and greatly simplified operations. A/B skip-stop services were only run during the day, from 6am to 6pm Monday through Saturday, because frequent headways were required to make the system work properly. If headways were too long, then the wait at an "A" or "B" station would become intolerable, annoying passengers. The experiment was deemed a success and was spread around the system.

A/B skip-stop service was implement on the North-South Route (Howard-Englewood-Jackson Park) and Ravenswood Route on August 1, 1949 as part of massive service revisions on the North and South sides. Twenty-three low-use stations were also closed at the same time. A/B service was inaugurated from 6am to 9pm Monday to Saturday. Unlike on the Lake Street Line, "A" and "B" stops were spread across the whole line. On the Englewood and Jackson Park branches there were no skip-stops -- all trains made all stops -- with all "A" trains sent to Englewood and all "B" trains sent to Jackson Park. Sunday A/B service was briefly tried on the North-South Route from noon to 9pm beginning January 1, 1950, but it proved to be unpopular with the public due to the longer intervals between trains and was dropped in 1952. At the same time, A/B service was abandoned on the North-South on Saturdays after 6pm and all but during rush hours on the Ravenswood. Although popular and operationally successful, the limits of A/B skip-stop service were already apparent.

The next line to get A/B service, albeit briefly, was the Milwaukee Route. Upon rerouting the line into the new Milwaukee-Dearborn Subway on February 25, 1951, the CTA instituted A/B service on the line, but it was annulled after just one weekday morning rush period on Monday, February 26! Apparently, there was insufficient time to change all the manually-operated destination roll signs on the cars when a train changed from an "A" train to be "B" train at a terminal. The Milwaukee Route, which only had a handful of "A" and "B" stations anyway, would return to all-stop service for the next seven years.

On December 9, 1951, the CTA instituted A/B service on the Garfield and Douglas routes. The Douglas operation changed quite a bit, with five stations closing, one new station opening, and eliminating short-turns at Lawndale. The branch was also truncated from Oak Park Avenue to 54th Avenue shortly after on February 3, 1952 (it was to have happened concurrently with the adoption of A/B service, but was delayed by a court injunction). The Garfield Line had fewer changes. All trains now operated from the Loop to Desplaines Avenue, with service beyond to Westchester replaced with buses. Laflin station was closed and service was withdrawn from the Wells Street Terminal. Interestingly, unlike most lines, a high proportion of stations on the Garfield Line were assigned to be "A" or "B" stops, with less than half being "AB" all-stops. A/B express service was operated during Monday through Friday rush hours only, with all-stop service at other times. In addition, Sunday patronage between Laramie was Desplaines was so poor that buses were substituted for trains in this section from 5am to midnight on Sundays. Skip-stop service proved short lived here, though. Station closures and changes to the A/B pattern occurred frequently on the Garfield Route to accommodate construction of the parallel Congress [Eisenhower] Expressway and replacement Congress "L" line. In fact, the required staging resulted in A/B service being abandoned altogether on the Garfield Line on September 20, 1953. Ironically, so many stations were closed on the Garfield Line at the same time that running times were virtually the same after A/B service was abandoned as before. Meanwhile, the Douglas continued running A/B service despite some station closures and a reroute to the Loop via the Paulina Connector and Lake Line in 1954.

Above: Car 2403, leading a two-car northbound train at Francisco on August 14, 1978, shows a Ravenswood All-Stop destinjation of the period. For a larger view, click here.
Below: A four-car train of
2400s stops at Belmont on August 15, 1978, showing a Englewood-Howard "A" destination of the period. For a larger view, click here.
(Photos by Doug Grotjahn, Collection of Joe Testagrose)

A/B skip-stop service was abandoned on the Douglas branch completely on June 22, 1958 as part of the larger service revision. On that date, the new Congress Line opened, replacing the old Garfield Park elevated. Trains operated from Logan Square on the Milwaukee Line via the Milwaukee-Dearborn Subway and onto the Congress Line to Loomis Junction. From there, alternating trains went to Desplaines on the Congress and 54th on the Douglas. The through-routes service was named the West-Northwest Route. A/B service was instituted on the West-Northwest, although all of the "A" and "B" stations were north of downtown. All "A" trains operated via the Congress west of Loomis and all "B" trains went on the Douglas, making those branches all-stop operations. A/B skip-stop expresses were operated Monday-Friday day and early evening and Saturday daytime on the Milwaukee portion of the West-Northwest.

From here on out, A/B service generally stayed the same. Over the decades, there were several changes to station types and stopping patterns, with some changing from "A" to "B" stops or vice versa and others going from skip stops to all-stops to meet passenger demand. There were also some modest expansions of the service: A/B service was run on the Dan Ryan Line when it was opened in 1969 and on the Kennedy [1970] and O'Hare [1983-84] extensions of the Milwaukee Line.

Otherwise, the only change to A/B skip-stop service was that the hours it was operated slowly began to dwindle. As mentioned previously, A/B service really only works with relatively high service levels. Once headways begin to spread, the wait at skip-stop stations begins to become unappealing for passengers in spite of the decreased travel time once on board a train. Although total trip time may not be increased, most passengers perceive waiting on a platform to be less desirable than a longer ride once on board a train. So as the CTA began to increase headways in the face of decreasing ridership and escalating costs, the hours of A/B service began to be rescinded.

By the 1990s, ridership had significantly declined over the years, in part because of city population shifts to the suburbs, and CTA service had been trimmed such that the gap between trains was fairly significant at some times of the day. On most lines, only the weekday rush hour period provided the kind of headways needed for skip-stop service. The CTA's standard for the discontinuance of A/B service had been whenever the scheduled gap between trains departing the terminal exceeded six minutes on any line, a policy designed to prevent any rider who just missed one train at a skip-stop station from waiting more than 12 minutes for the next one to arrive.

CTA officials began to remove the A/B system from portions of lines with wide stations spacing. On February 21, 1993, the CTA swapped the through-routes on the North-South and West-South routes. The resulting switch created the Red Line (Howard-Dan Ryan) and Green Line (Lake-Englewood-Jackson Park). On the new lines, A/B service was completely annulled on all parts of the Green Line, with all trains making all stops, and on the Dan Ryan service south of (and including) Harrison station where stations were roughly a mile apart. On October 31, 1993, the CTA opened on the new Orange Line, which has long distances between stations and a schedule with gaps between trains of more than six minutes for most hours of the day. Although A/B skip-stop service had been contemplated on the route when it was planned, it opened with all stations designated "AB Stations", in essence making the route all-stop all the time. And the Evanston (Purple) and Skokie (Yellow) lines had never had A/B service anyway.

By 1995, A/B service was running on very little of the "L" system. On February 5, 1995, the Brown Line (Ravenswood) abandoned A/B skip-stop service when the line changed to one-person train operation (OPTO). This left only the O'Hare branch of the Blue Line [former West-Northwest Route] and the Howard branch of the Red Line running A/B service, and even then only during weekday rush hours. And this last vestige disappeared on April 28, 1995, when these routes went to all-top service and the last of the A/B skip-stop operations disappeared.

Elimination of skip-stop service met with fairly positive results, according to the CTA . After the Dan Ryan segment of the Red Line became all-stop in 1993, some stations that had been designated "A's" or "B's" saw ridership gains of 34 to 50 percent, in part because they suddenly were served by twice the number of trains than before, officials said. Phaseout of the system also ended the inconvenience for people who boarded at "A" stations and must transfer trains to reach a "B" stop, they said.

But running times were also lengthened slightly after the elimination of A/B service due to increased running and dwell times. On the Howard leg of the Red Line, for example, it took trains up to two minutes and 30 seconds longer to complete the trip between downtown and the Howard terminal after all-stop service was instituted.

Currently, all seven "L" lines run scheduled all-stop service at all times. There are, however, some vestiges of the old A/B skip-stop service on the system, most notably in outdates station signage. Many column signs, or "symbol signs", still have "A Station" or "B Station" or the like along the bottom, although the CTA tried to paint over these words in the early 2000s (bored and vandalizing CTA riders subsequently scratched most of the paint off). There are also still many station name and symbol signs with blue, red, or green backgrounds. These refer not to the name of the line (Red Line, etc.) but rather to the old A/B stopping patterns, with blue representing "AB" stations, red indicating an "A" station, and green symbolizing a "B" station in the "KDR" signage scheme used from 1970 to 1995.