The 1500 N force would need to act in the direction opposite the train car's movement, so if we take the current direction it's moving in to be positive, we have by Newton's second law
-1500 N = (15,000 kg) a → a = -0.10 m/s²
If t is the time it takes for the train to come to rest, then
-0.10 m/s² = (0 - 5.4 m/s) / t
t = (5.4 m/s) / (0.10 m/s²) → t = 54 s
