It would be "Chess"
On six feet, I am a noxious drink,
Of whose effects you shudder to think.
Change only my second foot, and then
You convert me into a horrible den
Where the culprit, who gave the noxious drink,
Awaits the fate of which you shudder to think.
There is a tunnel. Inside is only one train track. A train comes in from the left at 2:00 and another train comes in from the right on the same track at 2:00 also. Both come out safely.
How was this possible?