This incident took place on the line between Littlehampton and Worthing, in West Sussex, earlier today.
It has already caused widespread disruption on trains heading to and from Portsmouth, with closures affecting stations up to Chichester.
Southern Rail tweeted about the collision shortly after 5pm, saying from its account it was ‘with great sadness’ it had to report of someone being struck by the train.
It has since warned those travelling by train tonight between Littlehampton and Worthing to expect delays of up to 30 minutes until at least 10pm tonight.
British Transport Police have launched an investigation.
Authorities have not yet confirmed if the person involved in the collision has died.