the train has (already) left the station

The window of opportunity to do, be a part of, or avail oneself of something has already closed. A: "Hey, I heard you can make a huge profit if you buy stocks in this company." B: "Nah, the train has left the station. It hit its peak a few days ago." Politicians are scrambling to avert an economic disaster following the collapse of the housing market, but most analysts are warning that the train has already left the station.