There are 2 tabs in the menu when looking at track rides and coasters. Under the Blueprint tab are prebuilt rides that are ready for placement and can also include scenery. Then there is the custom tab. This tab only gives you a basic station to that ride and its up to you to design the rest.

Sorry maybe my first post was misleading. I not meant coasters or track rides, i also not meant ingame editor to create such. I mean, can you create your own mod from scratch to build Rides like "Blackout" "Scizzer" "Kick-Flip" etc?

I'm not aware of any way of doing it other than for scenery only using things like basic shapes (unless someone knows otherwise).