What's the Name of That Book??? discussion

I’m trying to find a children’s book I read sometime between 2005 and 2015, but I don’t remember the title or author. Here’s what I do remember:
1. The story is about two kids — a boy and a girl (not necessarily siblings). The story is written in a light tone, even though it has some scary elements.
2. They get into a roller coaster cart (an old, abandoned roller coaster in a theme park), and the ride takes them through a portal or tunnel into a magical, strange world.
3. They travel through this world, of which the only thing I remember is that nearing the end of the travel they swim by boat and need to cross the right passage between the two gigantic pillars of water, connecting the sea and the sky.
4. At the very end children meet two witches — one of whom is blind. The blind one is a bad character, but falls in love with some ugly man, because she can’t see his ugliness (I don’t remember where he came from in the story) and she loses the mean spirit.
4. The cover of the book was illustrated/drawn and showed the children riding on the roller coaster (at least in the polish version, I think the author was not polish though).

If this sounds familiar to anyone or you have any ideas, please let me know! I’ve been searching for years and would be really grateful for any help.

Thank you so much!

Long shot, Measle and the Dragodon?

It was published in Polish as Lichotek i Dragodon.