Network Rail is to inspect high-risk trackside slopes across Britain following the Aberdeenshire crash which left three people dead.
The rail infrastructure body said it will use in-house engineers, specialist contractors and helicopter surveys to assess dozens of sites with “similar characteristics” to the stretch of railway where an Aberdeen to Glasgow service derailed near Stonehaven on Wednesday.
A landslip amid heavy rain and flooding is suspected to have played a part in the accident.
An inquiry has been launched by the Rail Accident Investigation Branch.
A union official said colleagues “thought the world” of Mr McCullough, 45, who leaves behind wife Stephanie and three children.
Network Rail boss Andrew Haines, who arrived at the scene on Thursday, said “my heart goes out to everyone affected by this tragedy” and paid tribute to rail staff and the emergency services “who responded so quickly and professionally”.
He went on: “Questions are inevitably being asked as to how this could happen and I am determined that we understand the circumstances that led to this devastating event.
“It’s too early to draw conclusions but it is critical that we investigate thoroughly and with care, and work closely with rail safety authorities to make sure this can’t happen again.”
Network Rail said it is working with meteorologists to strengthen the information it receives about flash flooding caused by extreme weather and its engineers are reviewing the remote monitoring of high-risk sites to test whether it can be improved.
Network Rail was warned about its resilience to severe weather just four weeks before the Aberdeenshire derailment.
A health and safety report by the Office of Rail and Road noted a spike in landslips, demonstrating the “vulnerability” of the railways.
Scottish Transport Secretary Michael Matheson, who visited the crash site on Thursday, said: “They (Network Rail) are well aware of our views about the need to make sure that we are taking forward the right types of mitigations that help to manage a challenge of these types of localised, intense weather events.
“I think one of the things we will see what comes from the investigation is whether the pace of that type of mitigation work needs to be stepped up. That’s not just a challenge across Scotland, it’s across the whole of the UK.
“I think it would be reasonable to presume, without unduly speculating, that weather had an impact in this particular incident.”
Mr McCullough was a former gas engineer who had been a train driver for seven years.
Colleagues told the PA news agency he was servicing the gas boiler of an Aberdeen train driver when they started chatting about the job and he decided to join the railways.
Kevin Lindsay, Scotland organiser for the train drivers’ union Aslef, said: “The tragic accident at Stonehaven has affected everyone in the railway family. Brett thought the world of his family, and his colleagues thought the world of him.”
Speaking on the BBC’s Good Morning Scotland radio programme, ScotRail’s managing director Alex Hynes said: “Yesterday was a devastating day for everybody who works in the rail industry in Scotland.
“Our love and support is sent to the victims of this accident and their families, those that were injured in the accident and anybody who was touched by yesterday’s terrible tragedy.”
The Queen, Prime Minister Boris Johnson and Scottish First Minister Nicola Sturgeon have also paid tributes to those killed in the “tragic” incident.