On board a real-life 'ghost train'
2 July 2012 Last updated at 00:34 BST
You won't see them every day, and they rarely run when you might actually want to use them - but they're beloved by rail enthusiasts.
"Parliamentary" trains - also known as "ghost" trains - are strange services which often run just once a week and in one direction.
They exist in order to keep certain lines open, because without them the train operators would often have to close the route - something which costs time and money.
But tracking these trains down - and taking a journey on them - is the particular passion of a select group of ghost train "hunters".
BBC News went along for the ride.
Video produced by Dan Curtis.