1:34 PM PT -- Train just released a heartfelt tribute to Charlie Colin saying, "When I met Charlie Colin, front left, I fell in love with him. He was THE sweetest guy and what a handsome chap. Let’s make a band that’s the only reasonable thing to do."

They continued, "His unique bass playing a beautiful guitar work helped get folks to notice us in SF and beyond. I’ll always have a warm place for him in my heart. I always tried to pull him closer but he had a vision of his own. You’re a legend, Charlie. Go charm the pants off those angels❤️🙏⭐."

Charlie Colin, one of Train's founding members, has died from what appears to be a tragic accident ... TMZ has learned.

The musician's mother tells us Charlie passed away after he slipped and fell in the shower while house-sitting for a friend in Brussels, Belgium ... but it's unclear exactly when he fell.

She tells us no one found Charlie until his friends returned home from their trip about 5 days ago.

Charlie had moved to Brussels where he was teaching a music master class at a conservatory ... according to his mother. She says he was also working at a studio and finishing making music for a film.

Charlie, a bass player, helped form the band in San Francisco in the mid '90s with fellow OG members Pat Monahan, Rob Hotchkiss, Jimmy Stafford and Scott Underwood.

They achieved mainstream success with their self-titled debut album "Train" in 1998 -- and were known for massive hits ... such as "Drops of Jupiter," "Meet Virginia" and "Hey, Soul Sister."

The band scooped up 2 Grammy's back in 2002 -- one for Best Rock Song and the other for Best Instrumental Arrangement Accompanying Vocalist(s). Charlie left the band in 2003.

His mother has paid an emotional tribute to her son ... calling him a beautiful human being.

He was 58.

RIP