The frontman briefly split from the band seven years ago to pursue a solo dream, but his album Last of Seven was not a hit - and he returned to Train.
But Monahan and his bandmates insist it was important for him to follow his ambition.
As part of a new Behind the Music documentary on Vh1, Monahan says, "It was so important to this band... The reason I did it was to escape... It basically brought the band back together.
"My solo record, as unsuccessful as it was, was the best thing that ever happened to the band because, by not being together, we realised how much being together meant."
However, bandmate Jimmy Stafford wasn't always convinced his singer's solo venture was a good idea.
He explains, "I went to see him in Vegas... and it was really difficult for me to see my singer up there with another band. And most of the show was Train songs. I was really upset, like, 'Man, if you're gonna do your own thing, do your own thing!'
"But looking back at it, I'm glad it happened; I'm glad he failed at it because I don't think that the band would be here and doing what we've accomplished the last couple of records.
"He needed to get away from us and do his own thing and just see for himself."