There's a light at the end of the tunnel. And it's a train.

This quip overturns the familiar reassurance “there’s light at the end of the tunnel” by revealing the “light” as an oncoming train. The humor is dark and deflationary: optimism based on distant signs can be dangerously mistaken, and apparent progress may conceal imminent harm. As a piece of gallows humor, it also functions as a coping mechanism—naming dread in a punchline when circumstances feel bleak or out of one’s control. The line’s durability comes from its compact structure and sudden reversal, making it a memorable way to express cynicism about false hope, institutional promises, or situations that seem to worsen just as relief appears near.

1) “There’s a light at the end of the tunnel—and it’s an oncoming train.”
2) “The light at the end of the tunnel is the headlight of an oncoming train.”
3) “I saw the light at the end of the tunnel… and it was a train.”