If it were a string connected to an spinning object, I would have a second thought, because for string there doesn't exist any normal force.
But for roller coaster placed on a rail, the rail acts as a surface so normal force does exist to cancel out the weight of the roller coaster. So I think given there is no other force besides the weight, the roller coaster won't be able to complete the loop at the very top.
I got confused about this when I was doing my homework. The question asks for the minimum velocity of the roller coaster at the top of the loop. The answer key says that the centripetal force is equal to the weight, mg, plus the normal force, even though the two forces are in opposite directions; to get the minimum force, I have to assume the normal force to be zero since no change can be made to the weight. I think the book gives the wrong term for something else than the true normal force.