To find the average speed of the train, you can use the formula:
\[ \text{Average Speed} = \frac{\text{Total Distance}}{\text{Total Time}} \]
In this case, the total distance is 210 km and the total time is 1.5 hours.
Calculating the average speed:
\[ \text{Average Speed} = \frac{210 \text{ km}}{1.5 \text{ h}} = 140 \text{ km/h} \]
So the correct answer is:
140 km/h