lots of differences in plot/scenario/characters/themes between the two, but i found they frequently gave me quite similar vibes, plus both are well worth your time (especially because neither are very long):
>> fast-paced horror/thriller genre
>> a group fighting for survival in a single confined space (apartment building vs train, for good portion of the movie)
>> 'infected' monsters whose main purpose is to kill humans
>> starts off very action-based, but become a lot more about people and their decisions as the storylines progress - huge amount of heart-breaking potential!
>> similarly, touching and very human moments amongst the bloodshed, as well as plenty of character growth/development
>> large cast of characters (some you'll love, some you'll hate) that will definitely end up shrinking... with brilliant acting all round
>> side-eying the military/questioning the bigger picture

gripping, complex & full of dark and futuristic themes, both of these dramas are absolute must-watches, particularly if you want to watch something special & unlike many other dramas
>> the execution of the dimension-switching in train is highly convincing, plus very reminiscent of the divided timelines in circle
>> similar overall vibes (although train is more supernatural? vs circle's scifi)
>> both focus heavily on investigations
>> the male leads suffer traumatic losses, dedicate their lives to protecting their loved ones & uncover some nasty truths along the way
>> ...with a side of thought-provoking (and often mind-blowing) moral/ethical issues caused by messing with fate (train) and memories (circle), and how these can butterfly effect to drastic proportions.