Rick Springfield -- Now a Subway Performer ... AND HE ROCKS!!!

Rick Springfield's career has hit an all-time low ... literally -- going underground Wednesday morning in NYC for an impromptu performance of "Jessie's Girl" ... on a SUBWAY PLATFORM ... and it's awesome.

It all went down during rush hour -- around 8:00 AM -- Springfield whipped out his guitar out of nowhere for a flash mini-concert, belting out three songs for busy commuters ... including the classic "Jessie's Girl."

And as a testament to how awesome it was ... a massive crowd of New Yorkers -- who don't stop for anything -- actually gathered around to sing along.