A £5,000 reward has been offered after vandals threw a concrete slab through the windscreen of a train.
British Transport Police (BTP) said they wanted to trace four Asian youths seen running from the scene in Lye, in the Black Country, on 1 June.
The 37-year-old driver, from Tamworth, Staffordshire, saw some youths on a footbridge before the slab hit the windscreen, injuring his arm, BTP said.
Officers also want to trace the driver of a Citroen who drove past the youths.
The operator of the train, London Midland, is offering the reward.
"We are offering this £5,000 reward in a bid to seek new information that will lead to a conviction and will do everything we can to assist BTP in identifying these offenders," a spokesman said.
"Incidents such as this are uncommon, but throwing stones at trains can have dreadful consequences and I'd urge anyone who has information to assist BTP in their enquiries to come forward."
The youths, aged between 15 and 18, are described as wearing dark clothing, with one in a hooded top with a motif on its front. Another was carrying a bag.