The IoM TT (Tourist Trophy) is one of the oldest and most difficult racing course in the world. It's also one of the most dangerous, which is why the FIM (the world road racing sanctioning body) took it off of the competition circuit decades ago. Even through they close the course to traffic, you may have noticed a tremendous number of "immovable objects" on the course. Every year, there are a number of riders killed on the circuit - nearly 300 since the first race in 1907 (though the majority of them are amateurs not actually competing.)
It's a very long course at nearly 38 miles with more than 200 turns, and riders are actually timed over 3 laps, being sent out at intervals, because actually racing each other would be far too dangerous.
Mark