Like others have said, the spiral mountain would likely take months to build and scenic...there was a father who built one for his family (on YouTube) that was very well done...I can only surmise it took weeks or months to build.
One quick idea for the proposal is to have a single line of track come into the living room or wherever you're going to pop the big question - have her sit down and then power-up the train and "deliver" the ring via the engine or train consist with either you or someone hidden with the remote control.
Just one tip: make sure she likes electric trains. Some women are turned-off by them, and the guys who play with them. My ex-wife is one of them 
