If someone goes off the rails, they start to behave in a way that other people think is unacceptable or very strange, for example they start taking drugs or breaking the law.
Mind you
- They provide good service. Mind you, they charge enough for it. (Eso sí...) - Mind you, they have already lost something of their impetus... (Eso sí...) - Mind you, do not lose more than you gain by [...] (Eso sí...)