Trainer John Moloney is to undergo surgery after breaking a leg in a trackwork accident at Caulfield.

Investigations are continuing after Moloney, who rides a lot of his team in work, struck a small gap in the rail that is used as a crossing by stewards.

Jake Norton, the Melbourne Racing Club's Head of Racing, said the gaps are between six to 12 inches wide with flaps on them.

"Evidently he's clipped his leg on one of those," Norton told Melbourne radio station RSN.

"He stayed on but has broken his leg quite severely and has gone off to hospital.

"He didn't fall and we just hope the break isn't too severe."

Moloney's brother, fellow Caulfield trainer Gerard, said he didn't know a lot about the incident.

"They say it's quite bad," Moloney said.

"I spoke to him briefly while he was in the ambulance.

"It's gone through the skin and there was a lot of blood."