The generic roller coaster vertical loop is the most basic of roller coaster inversions. Specifically, the loop refers to a continuously upward-sloping section of track that eventually results in a complete 360 degree circle. At the top-most piece of the loop, riders are completely inverted.
The next attempt at building a looping roller coaster was in 1901 when Edwin Prescott built the Loop-the-Loop at Coney Island. This ride used the modern teardrop-shaped loop and a steel structure, however more people wanted to watch the attraction, rather than ride. No more looping roller coasters were built until the Revolution opened at Six Flags Magic Mountain in 1976. In 2000, a modern looping wooden roller coaster was built, the Son of Beast at Kings Island. Although the ride itself was made of wood, the loop was supported with steel structure. Due to maintenance issues however, the loop was removed at the end of the 2006 season.
In 1980 at New Jersey's infamous Action Park, a loop was built into a covered water slide. Riders were warned to descend with their legs crossed and arms across the chest to traverse the loop safely (See picture on this discussion thread). Too many ignored this instruction, however, and either got stuck atop the loop (requiring the construction of an access hatch to free them) or injured themselves, or both. It was closed after a month, but remained on view for the remainder of Action Park's operations.
It has been dismantled since then and will probably not be rebuilt. As of 2006, no other looping water slides have opened at any parks. However, since 2002, the Swiss company Klarer Freizeitanlagen AG has been working on a safe design for a looping water slide and have now built and tested a 1:1 prototype of their design.
On the way up, from the bottom to the top of the loop, gravity is in opposition to the direction of the cars and will slow the train. The train is slowest at the top of the loop. Once beyond the top, gravity helps to pull the cars down around the bend. If the loop's curvature is constant, the rider is subjected to the greatest force at the bottom. If the curvature of the track changes suddenly, as from level to a circular loop, that greatest force is imposed almost instantly (see jerk). Gradual changes in curvature, as in the clothoid, reduce the force maximum (permitting more speed) and allow the rider time to cope safely with the changing force.
Obviously this "gentling" runs somewhat contrary to the coaster's raison d'être. Schwarzkopf-designed roller coasters often feature near-circular loops (in case of Thriller even without any reduction of curvature between two almost perfectly circular loops) resulting in intense rides -- a trademark for the designer.
It is rare for a roller coaster to stall in a vertical loop, although this has happened before. Turbine (then known as Sirocco) at Walibi Belgium once stranded riders upside-down for several hours. The design of the trains and the rider restraint system (in this case, a simple lap bar) prevented any injuries from occurring, and the riders were removed with the use of a cherry picker.