An investigation has been launched after two road rail vehicles crashed and injured two workers.
The Rail Accident Investigation Branch (RAIB) said the crash happened at about 07:00 BST on 2 May at Ramsden Bellhouse in Essex.
It said the vehicles had been used to replace overhead electrification equipment and were on the same line, travelling towards Wickford.
The RAIB said initial inquiries found no evidence of a fault in the machines.
It said the crash happened as the vehicles were travelling in a worksite between Billericay and Wickford stations at the end of a night shift.
The RAIB said one of the vehicles was a mobile elevated working platform (MEWP) and it was being followed by a long reach mobile crane.
It said two people travelling in the MEWP basket were injured, one seriously.
The RAIB said its investigation would check vehicles travel safely within worksites.
It would also determine if processes were in place to govern the hours trackworkers were on duty, the identification of fatigue and how rest periods were managed.
A spokesperson said: "Our investigation is independent of any investigation by the railway industry or by the industry's regulator, the Office of Rail and Road.
"We will publish our findings, including any recommendations to improve safety, at the conclusion of our investigation."