Transit, the 37-year-old photographer's latest project, aims to "capture the daily theatre" of Singapore's multi-racial passengers on board its Mass Rapid Transit (MRT) trains.

"If you commute on the MRT and we are forced two inches in front of the doors, we'd all have the same reactions and share the same expressions and vulnerability," he told the BBC.

"The MRT is special to me perhaps because I grew up beside a train station. I would watch the old trains pass by my window everyday," he said.

He was inspired in 2011 when he returned to Singapore after a stint in Nepal, feeling like it was "a different country".

"I found that the trains in Singapore had become so crowded that it was difficult to board them during peak hours.

Out of frustration, I started to photograph what I saw at the doors."

As a photographer, Mr Koo said he tries to "make it a point" to spend time with his subjects and environment.

But the nature of this project drove him to adopt a different approach.

"It's practically impossible for me to speak to the commuters during the photography process," he said.

So he took things a step further, and turned to social media.

Mr Koo's Facebook page, which currently has more than 5,000 likes, also sees users actively commenting on his photos and sharing them.

He has managed to identify 15 people so far, from his photographs.

"With social media, I realised that it's actually feasible to connect with the commuters I photographed," he said.

Among them, Singaporean housewife Teresa Lee, whose daughter Astrid got in touch after a friend spotted her mother's photo on Facebook.

Mr Koo also said that his Transit project has seen its fair share of negativity online.

He told the BBC that he had seen some passengers in his photographs being "victimised by online trolls and insensitive netizens", and highlighted an incident involving a commuter whose photo was "unnecessarily" attacked online.

He then took down the post

His photographs were also recently featured on an internet forum "without his permission", he says, and prompted more "unkind" remarks.

"I want to leverage social media for its reach. But I also want to avoid the damage it is capable of," he said.

"If it comes down to that, I'll offer to remove the photos, delete the posts and even put up public notices to give voice to [my subjects'] concerns."