A forgetful commuter left a bag of $30,000 in cash on the Long Island Rail Road — and got it back with the help of a clue inside and some good detective work.

Assistant Conductor Christopher Nocito discovered the hefty sum in a bag forgotten on a crowded LIRR train to Ronkonkoma after the holidays last year, according to ABC7.

“Did not expect to see that when I opened it,” Nocito told the outlet. “I didn’t actually count it.”

The bag was brought to MTA Police Detective Kristin Riker, who located a date book inside, with the owner’s upcoming appointments listed.

“I was looking through the bag to if there was a receipt, a name,” she explained to ABC7.

“I saw this person had an appointment to get their car inspected so I ended up contacting their mechanic and I actually sent their mechanic a picture of this person … he helped me identify who it was, and that got the ball rolling to contact Long Island Railroad lost and found and reach out to this person so they can get their bag.”

ABC posted the story on its Instagram page, and followers were incredulous over the bag owner’s negligence, with questions such as “How u forget 30k?” and “Why was someone walking around with 30k?”

“They are very lucky. There are still some honest people out there,” another added.

“They should have been guarding that bag with their life.”