My English textbook has question that I don't understand why they choose the correct answer -
(correct) There's a strike at the airport so we're going to take a train.
(not correct) There's a strike at the airport so we'll take a train.
Why did they choose the first sentence?
They are interchangeable in most cases. However, sometimes we may feel the difference between them. For example:
Stop,stop! We're going to hit the tree. I won't use "will" here.
When you use "be going to", it indicates something that has already started its movement like a rolling stone.