British Transport Police are appealing for information after a train struck a bicycle left on rail tracks in Bedhampton, causing a "blinding" explosion.
Officers have released CCTV images following the incident, which took place in the early hours of Saturday morning.
Inspector Mick Morriss said: “CCTV captured two youths riding across the level crossing at 1.34am. Seconds later, out of shot, one of the bikes is pushed back towards the crossing – we believe deliberately – landing on the tracks.
“Two hours later it was struck by the early morning 3.34am Southern service from Barnham to Portsmouth and Southsea, causing an explosion and sending a blinding shower sparks flying into the air.
“I simply cannot over-emphasise enough the dangers of this type of crime.
“Through sheer luck, no-one was injured and the train was relatively undamaged, but the consequences could have been far worse. Trains travel at high speeds and there is a very real risk when they strike objects on the track that they could be derailed.
“If you know who was responsible, or have any information that could assist our investigation, we want to hear from you.”
Nicola Dooris, community safety manager for Network Rail, said: "This was nothing short of mindless vandalism and those responsible clearly gave no thought for the impact their actions had on others.
"The incident caused delays for passengers and put innocent people's safety at risk. The railway is not a playground and we would urge anyone with information to contact the British Transport Police."
Anyone with information is asked to call Freefone 0800 40 50 40 quoting reference 83 of 10 March. Alternatively, contact the independent charity Crimestoppers anonymously on 0800 555 111.