Mind The Gap was designed by London start-up Hack Partners for the company, to keep employees safe at work.
The technology uses audio and Bluetooth signals to detect if users are in close proximity to one another.
Network Rail staff now use the app widely, and it's being rolled out to other companies.
"I am immensely proud of our entire workforce for the part they have played to keep the country running throughout the pandemic," Martin Frobisher, Network Rail's safety, technical and engineering director said.
"As we start to plan for a return to office working, we've been looking at absolutely everything to make sure we can continue to keep our people safe."
How it works
Employees can choose to download the app and set their desired distance, in line with government guidelines.
They will then receive a notification when another user of the app is too close.
Mind The Gap uses inaudible ultra-high frequency sounds and Bluetooth to calculate the distance between phones, so it does not require an active internet connection to work.
source: BBC News
A social-distancing app which alerts someone when a colleague is too close is being used by Network Rail.