It's the ultimate Welsh trivia question - where's Wales' underground railway?
London has the Tube, Glasgow and Newcastle the Metro - and Wales?
You can feel justly smug if you know the answer. Hidden in a corner of a slate mine in Blaenau Ffestiniog is a railway that gives you a steep, short journey down from daylight to the slate face.
The only problem is that the yellow carriage has been making that trip up and down ever since the late 1970s. From today, they're getting a crane in to lift it out - and send it off for an upgrade. If that sounds like a simple engineering process well, it probably isn't.
The journey's been one of the major links between what Blaenau was built on - and over - the slate industry - and the rest of the world.
Maybe a million or more passengers have plunged down the hillside to experience a much darker place - and the slate galleries descend for 14 levels.
It's done the journey a 170,000 times and travelled the mileage equivalent of a trip round the world, even though it's the same journey each time.
The man who piloted us down on one of the train's last trips says he loves the way that short journey is part of the process of giving yourself up to the mines. The doors slam shut, you feel quite cramped in the cabin, the whirr of a wire echoes around you - and you're lowered into a place that once employed thousands of miners.
It's going but not for good. By the Spring, the yellow train will be looking new and proud again. Until then, you get to walk down to the slate levels in the footsteps of the original miners.
Unlike London Underground's Northern line, there's no voice telling you to mind the gap - just to mind your head as you clamber into the carriage!