They want to speak to the driver of a tractor who is thought to have been close to the Beech Hill crossing on Springs Road, Misson Springs, around the time of the accident.
He may be able to assist police with their enquiries into the crash, after which four-year-old Emma Lifsey died as a result of her injuries.
Emma, from Haxey, North Lincolnshire, was airlifted to Sheffield Children’s Hospital but died in the early hours of Wednesday 5th December.
An inquest into her death will be opened tomorrow by Nottingham coroner’s office.
The car Emma was in was being driven by a 67-year-old woman who remains in hospital in a serious but stable condition.
Her injuries are not believed to be life threatening and officers will speak to her at the earliest opportunity.
The incident was reported to British Transport Police (BTP) at 12.35pm on Tuesday 4th December.
“We believe that a man driving a tractor may have been close to the crossing prior to the collision,” said Det Insp Mick Jackson of BTP.
“He may have vital information which could assist our investigations into this tragic incident.”
The police investigation is ongoing as team of detectives are look at how the vehicle came to be on tracks.
The train has been moved to a depot in Doncaster where it will be forensically examined.
The line re-opened last night.
Anyone with any information is asked to contact British Transport Police on 0800 405040.