The way I read it, LBNT is partly about Billie as a kid always wandering off and getting lost and also about this family he knew that was a kid in his kidsโ school. The parents were a lesbian couple and their child was born with indeterminate gender and they wanted to give the child a nonbinary name and identity until they could decide for themselves. Train might have been the name or it could be a made up name for the song based on the kid.