 From looking at this blue curve of position as a function of time, and the instantaneous velocities it implies, it seems the train slowed down and stopped at a station briefly before increasing its speed as it continued on its journey. The velocity curve is drawn in green, and just as slopes on the displacement graph gave us velocity, slopes on the velocity graph give us acceleration. So for this example we see that the acceleration is negative up until team 1, then both velocity and acceleration are 0 until time 2 when the train starts moving again and its acceleration is positive.