Both of the FL's in these dramas are badass.
The main conflict in both of these is the greed of the antagonists, and, both of the ML's are head over heels for the FL!

The same couple act in both of these dramas.
If you liked the historical aspect of either of these dramas, what the other one!

Both of these dramas are about seeing into the past or future.
They both have the same ML.
They both have the same exact endings as well.

While these two may not both have the same plot (one is about time-travel and solving a mystery while the other is about the abuse and kidnapping of a child) they both share the overlaying theme of maternal love.
The love that these mothers had for their children is enough to make you cry.

Time travel dramas about going back and forth in time for a loved one.
They're both equally heart-wrenching in different ways.
Alice is about maternal love.
Someday or One Day is about romantic love.

Both of these dramas revolve around a girl who lies about having a boyfriend. She takes a photo of a random boy and shows it to everyone, saying that he's her boyfriend. Unbeknownst to her, the boy in the picture goes to the same school as her and isn't the kind type.

Both are about the unrequited love of one character towards another character who they have known for a long time.

Both of these dramas have two characters who are intricately involved each other through parental ties as well as a mysterious murder case. In both, the leads travel into different dimensions for each other.
In Train, the ML travels into a parallel universe to protect the FL.
In W, the FL travels into the world of comics to save the ML.
Both of these dramas are beautifully shot and while Train does focus more on the murders and W more on romance, they both give extremely similar vibes when it comes from the transition shots from dimension to dimension.

Both of these dramas go over the loss of loved ones and the determination to meet them once again. Both of these dramas focus on a murder that happened, and both have a fantasy aspect.
Train is about a parallel universe with dopplegangers.
Someday or One day is about time travel and doppelgangers.
Both these dramas are beautifully made and both keep you on the edge with their plot twists while still managing to be huge tearjerkers.

Both of these dramas take a look at different adults and their different takes on romance. Both of these dramas focus the romance lives of a particular family as well as the past of the adults in that family.
They both give off a fun lighthearted vibe and both dramas have amazing chemistry.

While Once Again is about divorced people finding love again, Never Twice is about those who have been mistreated swearing that they will never let anyone mistreat them again.
Both of these dramas share a faminial and cheerful vibe while giving you tons of cute couples as well as focusing on a mystery that all the characters eventually become entangled in.

Both dramas feature the characters lives being torn apart through a traumatic past and multiple killings. The main focus of both this dramas is on two characters, one a detective, and the other who is a psychopath. In both of these dramas, the psychopath and detective have a familial relationship.