Final answer:
Negative acceleration indicates that the train is decelerating, which means its velocity is decreasing. Acceleration is a vector, so the negative sign is consistent with the direction it's applied in the chosen coordinate system.
Step-by-step explanation:
If a train enthusiast determines that a train has negative acceleration, we can conclude that the train is decelerating. Negative acceleration means that the train's velocity is decreasing over time, and this acceleration is in the opposite direction to the train's motion.
When velocity and acceleration have opposite signs, the object slows down, which is commonly referred to as deceleration.
For instance, if the train's initial velocity is positive and to the right, then a negative acceleration is to the left, which reduces the train's speed. It's important to note that acceleration is a vector and its sign should be consistent with the chosen coordinate system.