That's because they're the classic places for 'something' to appear. Like the grand piano that plays by itself, the human anatomical model that moves by itself ...

When it sets off the bell rings, "ding-ding". Thus 'ding-ding-train'.

It won't be long before he is up and about.

Hold on to the strap. The train will start to move soon.

George felt the train begin to move.