A disgusted passenger snapped a quick picture of Mcguire and posted it on Twitter in hopes that followers would help alert authorities.
Police learned about the Twitter alert, logged on, and within 24 hours Mcguire was behind bars.
Other subway passengers thought Twitter was definitely put to good use in this case.
"It's not surprising because people tweet about every little thing nowadays. 'Oh I did my laundry. Ok, well we'll tweet that. Oh, there's some guy touching himself. We'll tweet that,'" Mike Carrera, a Boston subway passenger.
"For all of the kind of junk out there, it's pretty cool that it could be used to, you know, to solve crimes," said another passenger Samantha Broadhurst.
The passenger who first sent the Twitter alert later tweeted that the real heroes in this story are the police officers.