The unknown. For example, in some stories, there are unexplained noises or disappearances. You don't know if it's a person playing a prank or something more sinister. It could be a supernatural entity or a criminal, and not knowing is what makes it so scary. In one story, a trucker heard whispering outside his truck at night, but when he looked, there was no one there. It sent shivers down his spine just thinking about what could have made that noise.
The unexpected appearances are really scary. Like the phantom hitchhiker suddenly showing up by the roadside and then vanishing when you stop. It gives you a shock.
Since I don't know the 'the horror truck story' well, generally in horror stories involving trucks, the large size and power of the truck can be a scary element. It could be used to run over people or chase them in a menacing way. Also, if there is something hidden in the truck like a monster or a dead body, that would add to the horror.
The idea of the ice cream itself being tainted or cursed is also quite scary. For example, if the ice cream is described as having a strange color, a bad smell, or making people ill after consumption. It turns something that is usually a source of joy, like an ice cream from a truck, into a vehicle for horror.
Haunted aspects are really terrifying. Like when there are reports of seeing ghostly figures in or around the truck. It gives you the creeps just thinking about it. Also, strange noises that seem to come from the truck when it's empty or not supposed to be making any noise can be very unsettling. Imagine being in a quiet place and suddenly hearing a loud bang or a screeching sound from an old truck.
The sudden appearances are really scary. For example, when a semi truck just shows up in the middle of nowhere, like on an empty desert road or a foggy mountain pass. It gives you no time to prepare for it.
There was a trucker who stopped at a truck stop. As he was sleeping in his cab, he woke up to see a shadowy figure peering through his window. He tried to start the engine to get away, but it wouldn't turn over at first. After a few tries, he managed to leave and never went back to that truck stop again. It was really terrifying for him.
One scary truck stop story is about a driver who heard strange knocking on his truck's door in the middle of the night at a desolate truck stop. When he looked out, there was no one there. But the knocking continued. He got so spooked that he drove away immediately.
I heard about a tow truck driver who got a call to a haunted house area. When he was towing the car away, the mirrors of his truck showed a face that wasn't there in reality. He blinked a few times thinking it was his imagination, but the face kept appearing. He sped up to get away from that place. It was really terrifying for him.
The lack of escape. A bus stop is a fixed location. You can't really run away easily if something scary shows up. In some stories, the bus that is supposed to be the means of escape is either late or doesn't come at all. This feeling of being trapped while something scary is approaching is one of the scariest parts of these bus stop stories.
One common element is overbearing behavior. Mentors might be too strict or critical, like constantly nitpicking every little thing the trainee does.