Final answer:
The low slung car carrier is most likely to get stuck on raised railroad crossings due to low clearance. For the bouncing trailer observation, a trailer that bounces slowly is likely nearly empty as there is less weight to dampen the suspension system's movement compared to a heavily loaded trailer.
Step-by-step explanation:
The vehicle most likely to get stuck on raised railroad crossings is the low slung car carrier. This is because they have a lower clearance between the bottom of the vehicle and the road, making them susceptible to getting caught or scraped on raised surfaces such as a railroad crossing. On the other hand, single-axle tractors pulling a trailer, single-axle tractors on a return journey empty, and empty moving vans typically have higher clearance and are less likely to face this issue.
When you observe a freight truck's trailer bouncing slowly on the highway, it is more indicative that the trailer is nearly empty. A heavily loaded trailer will have less bounce because the weight stabilizes it, while an empty trailer will bounce more due to the lack of weight and the springiness of the suspension system.