The driver of the car involved in an accident with a train on an unstaffed Suffolk level crossing on Sunday remains in a critical but stable condition in Addenbrooke’s Hospital.

Meanwhile investigators from the government’s Rail Accident Investigation Branch (RAIB) have been informed about the accident and are preparing a preliminary investigation.

A spokesman for the RAIB said it was not certain a full investigation would be necessary – but it was necessary to establish some basic facts.

He said: “An accident on a level crossing at Melton has been reported to the Rail Accident Investigation Branch. We are gathering evidence. This will inform our subsequent decision as to whether to conduct a full investigation of our own.”

A key issue is likely to be whether there was any call from Melton to the Network Rail control centre before the car started to travel across the crossing.

The crossing is one where drivers have to call a Network Rail control centre to get the all-clear before venturing out on to the track. The driver is understood to come from the Bury St Edmunds area.

Officials from both Greater Anglia and Network Rail said they were not able to comment on the cause of the accident while the RAIB investigators were about to start their work.

The RAIB is an independent body set up in 2005 by the government in the wake of the 1999 Ladbroke Grove rail disaster to investigate all accidents on the country’s rail networks – main line, London Underground and tram networks.

Its inspectors gather evidence that can be used in try to understand why accidents happened and how to prevent similar incidents in the future – but it cannot prosecute.

If the preliminary report establishes there was a clear cause of the accident that does not require a detailed report then it can hand over its findings to the appropriate authorities which could include the British Transport Police who might then decide that further judicial action is needed.

The actual train is not expected to be out of service for long – it did not suffer major damage and was not derailed. It was driven under its own power to Norwich where it is expected to be returned to service within a few days.