Well, it's a combination of things. Her unearthly silence for starters. She doesn't interact like a normal person. Then there are the strange occurrences that seem to happen around her, like the lights flickering. It gives the impression that she has some sort of otherworldly power or is associated with something supernatural, which is quite frightening.
Her appearance is one factor. Maybe she has a pale face, or strange eyes that seem to look right through people. Also, her behavior like not blinking or staring constantly at passengers is really creepy.
The fact that she is so out of place on the train. She doesn't seem to belong there, yet she is there among the normal passengers. Her actions, like whispering things that no one can understand and disappearing suddenly, all contribute to making her extremely frightening. It's like she represents the unknown and the things that go bump in the night, and that scares people on the train.
The best chapter pointer here is Chapter 75, "Master, I didn't mean to bump into you!". It opens with The ordinary people in the livestream were all stirred up by this scene. and closes on But, being a half-baked practitioner is just fine..
A practical answer is Chapter 88, because it turns the idea into a concrete scene. The chapter opens with She wanted to refuse, but didn't have the courage to do so., adds pressure through Jiang Yan immediately stepped forward, "I have one point, quick, quick,..., and ends on Let's go," Jiang Li.
A practical answer is Chapter 74, because it turns the idea into a concrete scene. The chapter opens with Having seen Jiang Li's ID before, he knew that this person named Shiguang..., adds pressure through It's not just a banner we are talking about;
Look to Chapter 54 for the clearest answer. It begins with Five minutes later, the people of Chixia Village, who had received the..., keeps building through Leading them was the village chief himself., and lands on She was also polite and friendly, and to top it off, she was good-looking,....
If you want the clearest chapter reference, go to Chapter 118. Early on, At 8 in the morning, several guests appeared on time next to the...; later, the chapter leans on Seeing the five individuals walking together in unison, Director Sun was....
Chapter 71 is the strongest chapter-level answer because it starts from Shu Yan curled her lips with a hint of jealousy, "When can it be my turn... and follows through on Now they had to work on an empty stomach..
If you want the clearest chapter reference, go to Chapter 88. Early on, She wanted to refuse, but didn't have the courage to do so.; later, the chapter leans on The Chen'an Entertainment she worked for was currently the largest....
Chapter 120 is the strongest chapter-level answer because it starts from Facing the sudden and somewhat strange request from Jiang Li, Director Sun... and follows through on Drink this." Wen Jiaojiao looked at the half cup of thick substance:....
Chapter 117 is the strongest chapter-level answer because it starts from Fragrant and cloyingly sweet potatoes and corn were out of the pot, and Qi... and follows through on Since entering puberty, his mother had exerted strict control over his....
If you want the clearest chapter reference, go to Chapter 113. Early on, The next day, Jiang Lili was up before dawn as usual.; later, the chapter leans on But this time was different; as she got dressed and opened her room door,....