What are the differences in meaning between the three following sentences:
The train leaves at 6:00.
The train is leaving at 6:00.
The train will be leaving at 6:00.
In practical terms, without more context, there is little difference between your three and bhaisahab's fourth, or even The train will leave at 6.00. Some coursebooks and grammars try to make us to believe that there is a significant difference, and that in any given situation, only one way of expressing the future is 'correct'. This is simply not true.
If you want to learn more about possible differences between the various ways of expressing the future, try this: