If you ask "What time...", you are expecting an answer with a specific time, such as 2:30 PM. If you ask "When is...", the answer could be more general, such as "early in the morning", "late in the afternoon"... The four first sentences are correct, although I would prefer to ask: "What time is..." The last two sentences are not correct. A better way to ask those questions would be: "When could I leave?" and "What time can I leave?"

If the train arrives today you can ask 1)"When will the train arrive?" Or 2)"At what time will the train arrive?" But if the train will come after a day or so then you should ask question number 2 only. Then ask further about the time.