The idiom "go off the rails" means to behave in a way that is unexpected, extreme, or out of control. It is often used to describe someone who has started behaving in a way that is reckless, irresponsible, or dangerous.
For example, if someone is normally very responsible and hardworking, but suddenly starts missing work and staying out late partying, you could say that they have gone off the rails. Similarly, if someone is normally very calm and collected, but suddenly starts getting into fights and causing trouble, you could say that they have gone off the rails.
The phrase "off the rails" comes from the idea of a train that has come off its tracks and is no longer following its intended path. In the same way, when someone goes off the rails, they are no longer following the path that they normally would, and are instead behaving in a way that is unexpected or dangerous.
Overall, "go off the rails" is a useful idiom to describe someone who has started behaving in a way that is out of character, and can be used in a variety of situations to express this idea.
• The project went off the rails when the team leader quit and left us without direction.
• My friend's life went off the rails after she lost her job and started drinking heavily.
• My diet went off the rails when I started eating pizza every day.
• The director's attempt to make a critically-acclaimed film went off the rails when the lead actor had a nervous breakdown on set.
• The politician's plans for reform went off the rails when he was caught in a corruption scandal.
• After the new CEO took over, the company's financial management went off the rails.