I uploaded editorial photos and videos from both metro stations and railwaystations to shutterstock. All photos and videos taken from inside the metro station were approved,
and all the photos and videos taken from above ground train stations were rejected as Non-Licensable Content even as editorial.
Can someone tell what's the legal difference between those two? Why underground is ok and overground not ok?

It's not my field of expertise, but the only thing I can think of is that your outside shots contain non-licensable content.
Maybe not even the metro station, but something else.

Some public railway systems are not suitable for commercial or even editorial shots.
For instance: parts of the Japanese railway system can not be licensed, also not editorial.

Of course, it's also perfectly possible that this is another example of pure randomness by Shutterstock reviewers.
Or maybe they decided that they already have enough shots of that place, and they just fobbed you off.

Just as an additional information: always check whether what you are shooting is not protected. Both commercially or editorially.
See: https://wiki.gettyimages.com/

SS is using AI to review images and has been doing so for about 3 years.