i dont wanna bring you down or anything,but a complex custom movement like sonic is a pretty advanced thing, and not the ideal thing to do when you dont even know where to start :p
its not impossible believe me, but it requires alot of thinking to get things to work like you want them to.
now i know how to do this, but its by no means for beginners. it requires a lot of complex stuff.
me and davios tank movement engine is a crude example of what youd need (its in "your creations" subforum)
im not trying to make it seem hard, its not the most difficult thing you can do, but what im saying is making a sonic style movement isnt where you should start, this is THE generic "i wanna make an RPG" dilemma.
anyways, back to helping you out lol
now if i remember sonic doesnt have real terrain finding like you wanted. it has terrain slopes which are (* is short for degrees) 0*,22.5*,45*,67.5* ect (they are in 22.5 degree increments which creates the illusion of smoothness), it doesnt have smooth terrain. so that means it does work somewhat like deadeyes example. ontop of these 22.5 degrees terrain things, it has quarter circles, and quarter pipes (for things like those rolling hills, and of course loops (although loops are a somewhat prescripted event.)) now all of these things are 100% possible in construct, and making a sonic engine that works excactly like the original is not as hard as i previously mentioned (but still somewhat difficult). if you understand Cos and sin and are pretty fluent in math it should be relatively easy.
like you can see here, the terrain is in 22.5 degree increments, the majority of the slope is 45*, then there is a 22.5 at the top, which transitions into a 0*, and at the bottom 22.5* which again transitions into a 0*
making a custom plat movement that uses sloped like this is pretty easy, but youll need to figure things out yourself, as its tedious and i dont want to explain it here
as for loops and hills, i have a .cap lying around that should make understanding how to do them pretty simple, but its on my other computer so i cant get it now.
anyways good luck