Might be because the 59 is a lighter route and runs less frequently than the 55, the 59 being up to every 20 minutes compared to 10 or less on the 55. But it's not just the 55A and 55N that are interlined with the 59. The 62H and 63W also are interlined with the 59, and all four of them are also interlined with each other. It's no accident that like the 59, the 55N and 62H have no Sunday. 55A is treated like a rush hour only variant of the 55N and has no Sunday service by default. So the interline that we're speaking of only applies Mondays thru Saturdays. Only the 63W has Sunday service out of the routes that run west of Midway. On Sundays and the big holidays that trigger Sunday schedules, I believe the 63W then gets interlined with the 63 if I'm remembering that right. That's easier to do because on Sundays and holidays the 63's Midway trips are every 20 minutes. The other half of the 63's trips on those days terminate at the Kedzie/63rd Place terminal used by the 52.