Later, after assuming control of the "L", the CTA introduced A/B skip-stop service. Under this service, trains were designated as either "A" or "B" trains, and stations were alternately designated as "A" stations or "B" stations, with heavily used stations designated as both – "AB". "A" trains would stop only at "A" and "AB" stations, and "B" trains would stop only at "B" and "AB" stations. Station signage carried the station's skip-stop letter and was also color-coded by skip-stop type; "A" stations had red signage, "B" stations had green signage, and "AB" stations had blue signage. The system was designed to speed up lines by having trains skip stations while still allowing for frequent service at the heavily used "AB" stations.