The 30-year-old track worker was struck by a train operated by South Western Railway near Surbiton station at around 11:35am on Tuesday 9th February 2021.
The train, which was travelling from London Waterloo to Salisbury, was travelling at around 76 mph on the down fast line.
The track worker was part of a group of four staff undertaking a planned inspection of switches and points. The work was planned to take place while the lines were open and with arrangements in place to protect staff from train movements.
The Rail Accident Investigation Branch (RAIB) said that its investigation would aim to identify the sequence of events that led to the accident. It will look into “the actions of those involved, the planned protection arrangements, the planning of the work relating to the inspection of switches and crossings where lines are open to traffic, and any relevant underlying factors”.