The work was first proposed by the high-speed division of UIC, and taken over by the working group "Improvement of passenger train performance on conventional lines". The idea was that it should be possible to improve the commercial speed on a given line, without too much investigation. One possibility was to increase the speed in curves without taking any other action, which will increase the lateral acceleration experienced by the passengers. Increasing the cant in circular curves was also a solution, using tilting train bodies was a supplementary possibility. What are the supplementary constraints on the passengers? The centrifugal forces are more balanced during the ride on the circular curve, but in the transition, a number of phenomena appear, and degrade comfort. On some occasions we know that onset of nausea can occur.