Translation:They will be taking the train tomorrow morning at six o'clock.
Both ways are fine here. You can view this event both ways in Swedish, either you focus on their intention, either it's more like a prediction. You can imagine somebody telling a story about all the things that are going to happen tomorrow, then this version would fit.
You could also say De tar tåget i morgon bitti klockan sex.
PS the yellow thing in your comment must be because you used the wrong apostrophe (doesn't bother me, just in case you wonder. If you use ' instead it won't look like that.)
it seems unnatural to me, http://spraktidningen.se/blogg/tidig-morgon-kom-fran-tyskan explains it's a shortened form of bittida
'bitti' stresses that it is really early in the morning (the speaker probably does not usually get up this early). "i morgon på morgonen" has another kind of stress, more like repeating yourself to clarify that you are really talking about 'the morning', not just about 'tomorrow' as the whole day.
Why isn't "get the train" accepted. Is that too colloquial for Duo?
I only ever use and have heard "take the train" when you're outlining the general transport you're going to use, "I'm going to take the train to London" Otherwise, "get the train/bus/tram" or "catch the train/bus/tram" is used.