Nice Idea

Why not? Then how can roller coasters do it?

It has nothing to do with the car keeping its self on the tracks, it is all centripetal force, the force of the tracks pushing you in while the cars angular velocity wants it to move in the direction of its velocity. Like when you spin a cup on a string, or even better, like a washing machine. So it seems to me that by the laws of physics it should work if there is enough momentum.

But I would love to know why you think it wouldn't work, if you have a scientific explanation (not sarcastic at all!). I'm curious.

Well i didn't say that physically it is impossible. All what you said about the centrifugal force is right. although with a heavy train like that i don't think a tiny struture like that can handle it.. but as i said.. it is completely possible with a careful robust design and a really high speed train to keep its weight attached to the rail during the flip.

There's no such thing as centrifugal force. It's centripetal you're thinking of.

It's unlikely it's real though; think of the danger and money involved in building something like this for a simple ad.

In basic physic problems it is true that one cannot speak of a centrigual force. This is the case when we use a static reference frame.

However, if the reference frame is a rotating one (sometimes usefull when solving space- physics problems), centrifugal force can be used