The incident happened near Meols Cop railway station at around 9.35pm on Wednesday, 18 March, as the train approached the railway bridge on Hawkshead Street.
Sergeant Stephen Lawman, investigating, said: “This could have been far worse, the fact that the driver escaped without any injury is astonishing. He was however, left extremely shaken by what happened.
“We would appeal to anyone who lives in the area to check their gardens or patios for any missing flagstones and to think back to whether they saw anyone hanging around at the time of the incident.”
The person who was seen to throw the slab is described as a large man wearing a black beanie hat.
Sgt Lawman added: “Incidents of this nature are, obviously, extremely serious. They pose a real threat to the life of staff and passengers, cause thousands of pounds worth of damage and result in delays to services.
“I would urge anyone who witnessed this incident, or who has information regarding the identity of those responsible, to get in contact with us. It is vital that the person who did this is apprehended as soon as possible.”
A Northern Rail spokesperson said: “Incidents such as this cost the rail network much more than a hefty bill. They can cause serious shock and upset to our drivers and conductors, and as a result, our passengers can suffer long and unnecessary delays and disruption to their journey.
“A damaged window also means a train has to be taken out of service for repair causing further delays and disruptions, something that is preventable and can be avoided.
“Each Northern Rail employee has the right to work in a safe environment and incidents like these not only put them in danger but also risk the lives of the passengers we serve.”
Anyone with information that could assist should contact British Transport Police on 0800 40 50 40 or text 61016 quoting reference PSUB/B6 of 19/03/2015. Information can also be passed anonymously to the independent charity Crimestoppers on 0800 555 111.
Contact us with your community, business or sport news. Phone 07581350321 or 07930717137