It can also be used to describe situations where some is constantly complaining about something over and over again
John: Wow dude you are really ridin the struggle bus today
Jack:Shut the frick up dude it wasnt my fault I hate you
John:....and here comes the complain train
When Jack hates his job soooo much and he tells everyone on earth about it like 50 times, Jack is riding the complain train.