Initial inquiries into a train crash in Wiltshire suggested the driver could not have done anything to prevent it.
Passengers were delayed when the high-speed train from London struck a fallen tree near Market Lavington on 10 July.
It was travelling at 90mph and the driver suffered minor injuries and shock.
The Rail Accident Investigation Branch (RAIB) said the driver had "little warning" of the obstruction on the line.
The 1306 service from London Paddington to Penzance was carrying about 200 passengers and four crew at the time.
A freight train assisted the damaged vehicle to Westbury where the passengers, none of whom were injured, were taken off.
A statement from the RAIB said there was no evidence so far that the railway, signalling, rolling stock or driving contributed to the crash.
It said: "The driver had little warning of the obstruction on the line and could take no action to avert the collision.
"The train was not derailed, but extensive damage was caused to the front cab due to a large branch entering its interior at the level of the windscreen.
"The RAIB's investigation is independent of any investigations by the safety authority."
The RAIB will publish a report, including any recommendations to improve safety, at the end of its investigation.