The train scene was filmed in Belgrade, Serbia. The train did not need any "make up" as the train operates in this condition regularly. The underground station was made for a metro line that never has been finished.
The film was originally produced by Universal for a nationwide theatrical release, but was instead offloaded to Netflix for an undisclosed amount.
Lizzy Caplan's (Alice) real life husband, Tom Riley, plays Chris the Wellness Centre Patient that warns Peter.
This is not the first alien invasion film Lizzy Caplan has starred in. She also starred in Cloverfield (2008) which was also was about an alien invasion but with one giant Godzilla-like monster taking over New York City.
This is Ben Young and Emma Booth's second feature film collaboration together after Hounds of Love (2017). Young and Booth have been friends for over a decade prior to filming Extinction.