Alvin and Dexter are an add on for the wonderful Ticket to Ride board game.
You can use them with any version of the game. They are a thwarty type thing you can use to stop your opponents from building train lines to particular cities.
I was kind of excited to see how they work so we had a go with them today. And they were promptly used by everyone else to stop me from building anything and making sure that I came a really poor last place. (at least, that's what I'm telling myself)
Oh well. There's always next time...