A train inspector was stabbed in the back after challenging two youths travelling without tickets.
The incident happened at East Tilbury station in Essex just before 9pm yesterday on the 8.20pm C2C service from Southend Victoria to Fenchurch Street in London.
British Transport Police said the ticket inspector was assaulted and stabbed.
The suspects are described as white and aged about 18. One had a black hooded top on, while the second wore a grey flat cap and a grey top.
Any passengers who witnessed the attack are asked to call British Transport Police on 0800 405040.