Q:

"The train will leave" vs. "is going to leave" vs. "leaves" vs. "is leaving"

From the grammatical point of view all are correct, just the meaning are different, please bring your clarification, thank you.

The Train will leave at 10:00 tomorrow morning.
The Train is going to leave at 10:00 tomorrow morning.
The Train leaves at 10:00 tomorrow morning.
The Train is leaving at 10:00 tomorrow morning.

A:

They're all just stylistic choices, with no difference in nuance of meaning.
The only context where "regular, repeat event" comes into play is when you say something like "The London train leaves at 8 o'clock" - if you don't specify any particular day, the implication is it does so every day (or at least, every week-day - it may leave at a different time, or not at all, on Saturday and/or Sunday).

