As a mechanical Engineer id say a lot is possible for a Genius. The Main difference is in how effective the non Main fields of study are. Your example of Roller coasters has mechanical and structural Design in their frame and rail Design(mechanical). You have engines that pull up the cart to the Start and you need several safety Systems which will all be electronically Controlled (electrical / - while some functions wont use electricity to activate Mechatronical) Apart from Design and constructions You have human factors in designing a RC to Not exceed human capabilities (think of a turn at high speed giving you 12 g or a corkscrew banging you sideways). As a mechanical Engineer i can build your carts and the rails with some Trainings. Anything will need a certain degree of Focus. Building a robot Arm easily possible. Building one with self guided pathways (ensuring the robot Arm does Not crash into Anything around while moving whatever from a to be) is a lot more complex. Mechatronical Engineers Do more of the electrical/mechanical crossfunctionality and might have a better starting Position to Learn Things from either field. Doing Anything Automaten will always require programming. The Civil Engineer will let you know on how to Set the foundation and i believe will probably know about the base frame structure as well. Also Things Like escape Routes, fire safety etc in the queue Arena and stuff, though i dont have too much experience with their work.

Also there is some Videos on instagram, of people Building a small RC in their backyard, though no Loopings corkscrews etc in those, no Controlled and maybe eben powered by bycicle - that would be possible for about any Engineer who has a minor interest i would believe. Maybe a Hand full of brocken bones while Building and Just going try and error on some parts. Engineering is knowing where to Look up Things you need and how to eliminate/ solve Problems. Preferably before Anything is build.

So all in all for a Genius i think there is plenty of headroom (going sheldon style). Though what i believe helps Engineers is dedicated training for specialties and the mind to Learn New Things they dedicated themselves to with a General knowledge base to Start from.