Old trains that aren't used anymore.

[Calling it the train graveyard is a bit strange, she thinks, but it is what it is. She doesn't pick the names around Midgar.

And, as if knowing exactly what Sakura is thinking, because she would wonder the same if it were her, Tifa goes on to explain as they swim along.]

They say that it's where the lost children go. So, I guess it wasn't so bad, no. It could've been worse.