I wish I had a decent sized back garden/yard - as I have access to fabrication equipment and bending/forming presses, easy access to a supply of steel and the equipment to handle it all. A huge list of suppliers (that my company has an account with) of all kinds of equipment like electo magnetic motors, hydraulic motors, roller coaster lift grade chain, bearings etc. I have powerful design software and the ability to use it to design a fully fledged coaster. I have access to and full use of fairly large transportation that would allow me to transport pre-fabricated sections up to 4m long and 2m wide.
It genuinely wouldn't cost me all that much to set out on a steel backyard coaster project if I had a decent sized garden to fit something in. Something like a drop, hill, 180 degree turn, hill, 180 degree turn back in to the base of the lift.
The only real problems would be planning permission, as the local council would be all over me about it
I think I'll stick to just making models for now though...