Explanation:

1. Moving with a constant speed:
 
When the passenger in a train moving with a constant speed tosses a coin, the coin will fall back to the spot where it was tossed from as if the train was stationary. Because the coin is also moving with constant speed along with the train.

2. Accelerating:
 
When the train is accelerating it changes its speed (increases),on the other hand the coin still moves with the same speed as before .So when the coin is tossed, by the time the coin reach back the train would have moved ahead  and so the coin will fall behind the spot from where it was tossed.
 
 
3. Negatively accelerating:
 
If the train was slowing down (or moving with negative acceleration) .On tossing, the coin would land ahead of the spot from where it was tossed. Because the coin moves with the same speed while the train slows down