There are some mild innuendos, but nothing too bad.

Book One only features violence when a recurring character attacks others with lasers. Book Two contains more violent moments, like head-butts and stylized injury detail, albeit still disturbing.

Season 3 in particular contains scenes that may be disturbing.

There is no red or human blood shown. But gray blood from otherworldly creatures is sometimes shown in rather graphic ways.

The series features some violence in the form of fist fights and battles with fantasy weapons. These scenes are usually brief though.

At least one use of "suck" in the Season 2 premiere.

Name calling, threats and other verbal put downs are commonplace.

A saloon bar is seen in book 4. However, the bar sells tea rather than alcohol. The saloon is only shown due to the characters being in an "old west car"

The show regularly deals with mature themes including, but not limited to: divorce, grief, trauma, depression, abuse, denial, peer pressure, bullying, child neglect, control, trust, empathy, betrayal, abandonment, identity and death. These issues are always handled in a serious and realistic manner and aren't played lightly.

A disturbing robotic monster is seen attacking the main characters several times throughout the series as well as frightening undead roach-like monsters that can devourer a person's soul.

Book Two doesn't have much emotional situations, but still has scenes of threat and horror. Some initial episodes have only mildly scary bits (like a deer that constantly shapeshifts in bizarre ways) while later episodes are more intense. Bullying, peer pressure, and social acceptability are reoccurring themes in this season, but unlike in Book One, they aren't the focus.

In book 3, two main characters are killed off. One in episode 5 and the other in episode 10. Their deaths, while brief, are somewhat graphic and very distressing.

The series has a lot of suspense, mystery and mild horror. The train essentially kidnaps people and thrusts them into dangerous and unpredictable worlds with life or death situations that they can't easily escape from.

The main characters (who range from young children to older teens) are frequently in danger and not everyone makes it out alive.

The intensity, subject matter and maturity increases with each season, with book 3 currently being the most mature and intense.

There are many scary sequences, which become more and more intense as the show progresses.

The series is overall very dramatic, intense and suspenseful (especially the second half of book 2 and all of book 3). Some younger viewers will certainly find several scenes upsetting, scary or just stressful.