A railway maintenance vehicle came close to hitting a one-tonne metal pile which had been left on the tracks.
The incident happened last month at Kirkham.
The Rail Accident Investigation Branch was called in to examine the circumstances of the near-collision, which happened while the line was closed to trains.
A spokesman for the RAIB said: "At around 5.15 hrs on May 15 a railway maintenance vehicle, which was moving in an engineering possession, stopped short of a large steel pile on the line.
"The pile, weighing around 1 tonne, had been left there on completion of earlier engineering works.
"We have undertaken a preliminary examination into the circumstances surrounding this incident."
RAIB officials have decided not to carry out a full investigation into the incident. Officials are expected to publish their conclusions and any safety recommendations soon.
Trains were due to start running on the line an hour after the incident was reported, the first service departing Blackpool North at 6.19am
Network Rail said it would be carrying out its own investigation into the incident.
A spokesman said: “At Network Rail safety is our highest priority.
"We were immediately notified about the incident on 15 May.
"A full investigation is underway to understand the factors which contributed to this.
"This includes liaising with the Rail Accident Investigation Branch.
"Once the investigation is complete we will ensure clear actions are agreed to stop this happening again.”
The line between Blackpool and Preston is currently being upgraded.
The work, to allow electric trains to run along the route for the first time, involves overnight closures.
As part of the scheme hundreds of metal piles are being driven into the ground. They will provide the foundations for masts carrying the overhead electrical wires.
The route will close completely for 19 weeks from November 18.
Electrification work is due to be completed by May next year.