Next time you're in Mauritania, Africa, don't forget to take a ride on what is probably the world's longest train, measuring in somewhere around 3km long (1.8 miles), depending on cargo load.
For around $5, you can travel from Nouadhibu to Zouerate (about 700km, 434 miles) which takes about 12 hours, according to this site. Apparently they've just recently installed passenger cabins on the train, so you don't have to ride on top anymore. That's a plus.
If you don't plan on visiting Mauritania any time soon, but still want to see a world's longest train, head to Hamburg Germany's Miniatur Wunderland where the world's longest model train rests.