[atoav]

Imagine a train network operator who says: we cannot say how long a trip will take with 100% accuracy, therefore we are not going to tell you at all. And evwn more evil: on the trains they blind the windows and rob you of every clue that would tell you where you are and how fast you are going.

This would be pretty uncomfortable.

What train riders (and users) are interested in is how long a thing should take in principle as well as clues about whether they are on track. When reality differs from that it is ok, as long as the ETA is somewhat in the ballpark area of the actual time of arrival.

[masswerk]

On the other hand, time tables should provide actual data in linear time (not some "apparent time" – "You'll arrive in 'just fine' at your destination."). I don't know the extent of the download, but, if not not substantial, it should just add a small offset prior to the main operation. Otherwise, if you're downloading a huge database, you can draw estimates from progress and display a sub-progress of this first task. (There are APIs for this).

In the now gone days of usability, it was considered good practice to annotate the progress bar of a complex task by displaying textual information regarding the sub-task and the progress made. This could be considered here as well. (E.g., "Loading data, estimate: 3.4 secs.")