This riddle has been around for some time on internet platforms such as Instagram, Facebook, and WhatsApp, and many people still argue over what the answer might be, as there are many possibilities. The correct answer is For the train to pass completely through the tunnel, it must travel 2 miles. After 1 mile travel, the train would be completely in the tunnel, and after another mile it would be completely out and since the train is traveling at 1 mile a minute, it will take 2 minutes to pass through the tunnel.