This is an idiom about loss of control of life and emotions.
A train runs on rails so when it goes off the rails that's very bad.
We can say, 'she's gone off the rails.'
We could also say, 'she's lost the plot.' The plot, being the story line in a film or play.
Maybe she needs some therapy.
In an interesting conversation recently, a lady explained that her sister was in rehabilitation. Oh dear, I thought - poor lady. Then after further questions it became clear that her sister was a physiotherapist at a physical rehabilitation centre!
Here's someone who's gone off the rails once or twice; one of my favourite mixed up people with a song called Rehab.
Two choices here:
1) The official video. Embedding disabled so you just need to click on the link. Worth watching - it's a great video.
2) No pictures but the song and lyrics - useful.