When teaching the future tenses it’s very common to talk about ‘present tense as scheduled future’. For example, ‘The train leaves at 7.30′. Personally, however, I think this might be somewhat misleading. The present tense is the present tense. Importantly, this is the eternal present. I believe, then, that the trains in ‘The train leaves at 7.30‘ and ‘The train will leave at 7.30‘ actually refer to two different trains. The first is an idealized, scheduled train that may, or may not in the case of Sydney trains, actually exist. The second, however, refers to an actual train.
Here is an advertisement from the BNC:
A special ” Victorian Belle ” period costume evening train will also be run over the line on Saturday July 4 headed by ” City of Truro “. Dining accommodation for this train is already fully booked, but ordinary seating is still available for this 40-mile special run, price £6.00 per head. A commemorative headboard will be carried featuring the Taunton 150 logo and the train leaves Bishops Lydeard at 19.15. Trade stands, ephemera stalls and a real ale/cider bar tent, including a limited anniversary beverage, will also add to the carnival atmosphere of this event.
Here we can see both the primary present and future used, and I think there is a subtle difference between them. The first is referenced by a long noun group describing an actual train, ‘A special ” Victorian Belle ” period costume evening train’, while the second, ‘the train’, refers to the scheduled time that this particular train is supposed to leave. Notice, also, that all other references to the event itself carry future tense.
So as I said I think the ‘simple present describing future event’ is a bit misleading. It is a future event but that does really help explain why we might use that form and not use future ‘will’. Another common example is ‘I catch the 4.30 train tomorrow‘ but I think this actually carries a meaning of ‘I am scheduled to catch the 4.30 train tomorrow’ so we are, in fact, talking about the present state of the schedule and not really the future event.