By Friday morning the clip had notched up more than 90,000 views since being posted on Wednesday and was number eight on YouTube's trending list.
The video published by police officer Carl Emery shows him in his police uniform playing John Lennon's Imagine while being largely ignored by passers-by – although he does get a round of applause at the end.
“It was completely spontaneous,” the law enforcement officer said of his impromptu recital, according to 20 Minutes.
Emery's police duties are to ensure safety at Geneva's Cornavin station, one of Switzerland's biggest.
But spotting the piano, he saw an opportunity for some light relief.
“I couldn't resist it. I had to give the commuters this pleasure,” he said, adding that he saw this as part of his responsibility as a police officer.
He said he had no idea his video would gain such a huge global following.