What do you do when you can go over it, can't go under it and can't go round it? That was the question for the engineers of a railway line in southwest China, but they've since come up with the answer: you just go through it.
The problem lied in the very limited space in Chongqing, which is one of the fastest-growing cities in the world. More than 8.2 million people are packed into the city centre, which is less than 500sq km in size.
So it's difficult to make the room to lay train tracks - especially with high-rise buildings all around - and when the engineers were really pressed for space, they came up with an idea: to put a railway line through a block of apartments.
While it seems totally crazy and you'd expect it to make life for residents of the 19-storey building unbearable, it's anything but - the building is fitted with noise-reducing technology that means the train is supposed to make about the same amount of noise as a dishwasher.
In addition, those living in the building can actually catch the train from that point on the track, with those on the sixth floor able to jump on from practically just outside their front door.
The city, which currently has about four metro railway lines, is expecting to have 18 metro lines in operation to deal with its vast population by 2050.