MIT students build a roller coaster in one week 12 years ago
They had people manually pushing it back up the last two humps, and then they had a rope they hooked to the cart to pull it up the last hump.
Source: was one of the people pushing it back over the humps. Surprisingly a sketchy job - you have to really go for it to get it over the middle hump, and if you don't make it then you have to bail really quickly.