LIKE CHARON piloting his lifeless charges along the River Styx, a darkened Metro train rumbled through the Tenleytown-American University Red Line station early last Tuesday morning, its windows black, its destination unknown. Although it never stopped for the passengers waiting forlornly on the platform, it did emit an ear-splitting horn blast.

We don’t know if the horn was meant to scare commuters, greet them, jolt them awake or warn them (gratuitously) not to try boarding a moving train. To passengers, it was just another bewildering, maddening, soul-sapping Metro moment, a quotidian annoyance barely worth mentioning.

Except that it is worth mentioning that Metrorail is a slow-rolling embarrassment whose creeping obsolescence is so pervasive, and so corrosive, that Washingtonians are increasingly abandoning it. Even as ridership climbs on MARC and VRE commuter trains, and holds steady on Metro buses, passengers are deserting Metrorail in droves.

Over the nine months ending in March, ridership slumped by almost 5 percent, or about 8,000 trips, compared with the same period a year ago. Officially, Metro blames the effects of sequestration. But in a region whose population continues to grow, the exasperations of using Metrorail are undoubtedly a factor prompting passengers to flee — in some cases to bike shares.

The comatose escalators; the crumbling ceiling at Farragut North, year after year after year; the funereal lighting; the frequent signal problems; the routine single-tracking that makes weekend Metro use torturous — all of this takes a toll on riders that Metro officials too blithely dismiss.

A case in point: The Washington Examiner recently reported that Metro has failed, badly, to meet its schedule to repair scores of elevators and escalators. The reasons are varied, but mismanagement is certainly a major factor. And management’s reaction? Officials told the Examiner that it considers its repair program to have been a success — despite the fact that just 78 percent of planned jobs will be completed on time. In school, that’s known as a C, but then, schools don’t grade on the Metro curve.

Last Monday morning, all five Metrorail lines were beset by mishaps, the second such one-day calamity in three weeks. The Red Line seized up for a half hour (steam on the tracks), the Orange Line for nearly as long (signal problems) and the Green Line for 10 minutes (downed track circuit). A door malfunction afflicted riders on the Blue and Yellow lines, some of whom were herded off a Huntington-bound train at Braddock Road.

A Metro spokesman, Dan Stessel, deigned to recognize that the Red and Orange line problems were “worth noting,” but sniffed in an e-mail to The Post that the other snafus were simply “the minor rigmarole of any AM commute.”

We suppose “minor”is in the eye of the beholder. But to commuters who depend on Metrorail, it’s all become a major pain in the neck.