An e-fit of one of the suspects was released by British Transport Police

The guard was punched and kicked after a dispute with five Asian men on the 2206 GMT service from Bradford Forster Square to Keighley on 13 November.

He was taken to hospital suffering cuts and severe bruising, police said.

News of the attack comes after rail workers demanded more be done to tackle violence on trains and at stations across the north of England.

British Transport Police have released a picture of one of the suspects and urged anyone with information about the attack to contact them.

Northern Rail is offering a £1,000 reward for information that leads to the arrest and conviction of the those responsible.

Zero tolerance

Det Con David Williams said they took attacks on rail workers "extremely seriously".

"Assaults on staff, physical or verbal, will not be tolerated and as one of our key policing priorities, BTP will arrest and prosecute those who commit such crimes," he said.

"It is vital that we work together to create and maintain a safe environment for both staff and the travelling public."

Earlier this month, the RMT union said staff at Northern Rail, which operates services across the region, were calling for a zero tolerance approach to violence.

Demands include more staff, an increased police presence and greater support from managers.

In response, Northern Rail said it was bucking national trends and had reduced assaults compared to 2005.