According to BTP, officers were called to the track 'close to Chichester station' yesterday (Sunday) at 5.20pm.
A spokesman added: "Officers attended the scene and recovered a bike frame. There are no reported injuries and no damage was caused to the train or the tracks.
"Placing items on the tracks is extremely dangerous and could result in serious injury or even death.
"We’d urge people not to trespass on to the tracks, you are putting your life in danger."