That stench on your CTA bus? That puddle of urine? Turns out riders aren’t always the ones to blame.

The Chicago Transit Authority has disciplined three bus drivers who were caught relieving themselves on their routes in the past few years, according to CTA records obtained by the Chicago Sun-Times.

In one bizarre, on-duty incident in September, a driver defecated on his bus after pulling over — busted by onboard surveillance camera footage.

The driver told his bosses he couldn’t hold it because he’d eaten “bad tacos.” CTA officials didn’t buy his story, according to a transit source who says the incident appeared to be “premeditated.” The agency initiated termination proceedings.

The source, who works for the CTA, says the official records don’t come close to revealing the extent of the problem, saying it’s fairly common for bus drivers to urinate or otherwise let loose on or near their buses and let the blame, and cleanup, fall to others.

“I can tell you it’s dozens we’re aware of,” the source says, adding that incidents are often ignored by supervisors or “classified as something else” in paperwork to obscure the offense. “This happens frequently, honestly. . . . There’s really no good excuse for it.”

On some occasions, horrified bus riders have witnessed the illicit potty breaks, according to records and interviews.

If punished at all, the offending drivers typically have been allowed to keep their jobs, though public urination and public defecation are against the law.

The most recent incident occurred the morning of Sept. 16 on the No. 21 Cermak route, records show. With no passengers on board, the driver pulled to a stop, walked to the back of his bus, pulled a plastic bag from his back pocket, dropped his pants and unloaded.

Surveillance images show the driver “then tied a knot” and “took the bag and the contents of the bag to the front of the bus . . . then twirled the bag around and attempted to throw it out the window, but was unsuccessful,” according to the CTA records.

“The bag full of feces and wipes hit the top of the interior of the bus and . . . it seems as though the feces must have gotten on the seat and the driver’s area of the bus,” the records show. The driver “tried to clean up the feces by wiping the seat and his shirt, he also attempted to pour the contents of a coffee cup on the seat to clean the area.”

He didn’t do a very good job, though. When the driver for the next shift got on to take over, she detected a foul odor and asked him about it. He blamed “an unknown customer that was on the bus earlier . . . with feces on his shoes,” records show.

She soldiered on until Cermak and Central Park, where “she curbed the bus, did a visual inspection and notified control that she discovered feces on both the seatbelt and top left corner panel inside the bus,” records show. Too “upset to continue,” she halted the run.

When the male driver was confronted by his bosses, at first he blamed “a homeless guy” for boarding and “throwing a bag of s—” at him, a source says.

The driver later told the “bad tacos” story and apologized. He was targeted for firing for “behavioral violations,” “conduct unbecoming” and “gross misconduct.” The Amalgamated Transit Union Local 241, which represents CTA bus drivers, says he has since resigned.

Under the transit agency’s rules, that means he could apply for another CTA job in the future.

Another driver was recommended for firing over a June 2017 incident in which records show he stopped his No. 20 Madison bus just before 5 p.m. at Randolph and Columbus and was spotted by a nearby supervisor “urinating from the rear doors of the bus.”

Later, a review of surveillance footage showed he was urinating against the rear doors, from inside the otherwise-empty bus. The footage, obtained by the Sun-Times, shows the driver nervously looking through the windows as he’s peeing.

The driver’s initial explanation: “There was a car parked illegally in the bus stop,” records show.

Later, he told CTA officials he’d been taking “water pills” and “couldn’t hold it.”

After meetings between union and management representatives, his discipline was reduced to a three-day suspension.

About a year earlier, a CTA customer “stated she observed the operator” of a No. 72 North bus “urinating outside the front door of the bus while at a red light” around 5:40 p.m. at North and Mobile, records show, though they don’t make clear what happened after the complaint was made.

In 2015, another bus driver got a one-day suspension for stopping his bus and peeing. A complaint — apparently from a rider or passing driver — described what happened: “Your bus driver on the 157 route southbound at about 4:30 p.m. on 4/20/15 pulled over under the viaduct near Rockwell and Ogden, got out, walked to the back of the bus and relieved himself in the street. That is gross, unsanitary and just wrong!”

The driver later told bosses it was “urgent” that he go, “and he did not want to dirty his uniform.”

CTA spokesman Brian Steele says such incidents are rare. He disputes the CTA source’s assertion that there are many more occurrences beyond the records that were turned over.

Steele says his agency provides numerous opportunities for bus drivers to find and use a bathroom while they are working. The CTA has agreements with businesses along bus routes to let drivers stop and use their facilities, and the agency also has placed portable toilets in select spots, he says.

“This kind of conduct by CTA employees is unacceptable and completely avoidable,” Steele says. “CTA does not condone actions like these, which certainly do not reflect the high level of professionalism shown by the overwhelming majority of CTA employees.”

Last year, the union raised a stink about the quality of those porta-potties, saying the conditions in them were so bad that some bus drivers were resorting to wearing disposable diapers.

ATU Local 241 president Keith Hill says there’s also a problem of inadequate “recovery time” at the end of routes for drivers to use the facilities.

Drivers are allowed to contact the CTA control center mid-route and ask for a “personal” break and, when they are given permission, to pull over and run in, say, to a business or a police station to use a bathroom.

“I do preach that, if you can’t hold it, call CTA and ask for a break,” Hill says.

The CTA uses “progressive” discipline that it has said takes into account the entirety of an employee’s service record.

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