A 70 metre section of brick wall collapsed during the work but no-one was injured during the incident on Christmas Day.
Network Rail is investigating the collapse which started when a small section failed. CCTV footage of the incident was posted on social media.
Are we doing enough to ensure engineers on the ground are safe to carry out their work?
— Construction Sport (@construct_sport) January 2, 2021
The wall was connected with a handrail bolted through the bricks which pulled down the longer section.
Network Rail said the original collapse was not caused by being struck by machinery.
Work was taking place with a digger to remove old rails and sleepers.
Train services in the area are still disrupted while Network Rail rebuilds the wall.
Mark Killick, Network Rail Wessex route director, said: “I want to pay tribute to the hundreds of staff who worked between Christmas and new year to improve the railway for passengers.
“I want to thank passengers for their understanding while the work has gone on, and also the communities where our teams have been working 24/7, as we know it can be difficult to have so much work going on next to your home.
“Unfortunately, during the holiday period our people also dealt with two unexpected incidents. Following Storm Bella, a landslip meant we needed to deliver emergency work close to Wanborough station.
“Also, while the work at Nine Elms was taking place, a section of brick wall on a viaduct collapsed, and there will be some changes to South Western Railway services from Waterloo as a result.
“We are rebuilding the wall as quickly as possible; there were no injuries and investigations continue into the cause of the collapse.”
Network Rail had made a promotional video on the same day before the incident.
…unfortunately on Christmas Day we had a mishap at the site, and the impact will be felt beyond just this week. While we will be handing the railway back on 4 January, we will be missing one track (of 8) for the reasons in this statement. ⬇️ /2 pic.twitter.com/C1DUjc0TE7
— Network Rail Wessex (@NetworkRailWssx) December 31, 2020