When a body slows down, its speed decreases. If it is traveling in a straight line, like a train on a straight track, we can also say that its velocity is decreasing. Acceleration is the rate of change of velocity or,
Acceleration = dv/dt
where, v is the velocity and t is time. Generally, we use acceleration in reference of increasing velocity. That is, an object is said to be accelerating if its velocity is increasing. For the scenario of decrease in velocity, we often use the term 'deceleration'.
Thus, in this case, if the train is slowing down, we will refer to it as a case of deceleration or negative acceleration. If the train was accelerating, its velocity would be increasing and it would speed up.
Hope this helps.