I will be in Salzburg on the morning I must travel to Ljubljana to join a R. Steves tour. I checked on-line for trains and it appears that there is only one train to get me from Salzburg before our 5 pm meeting time in Ljubluana. (Which seems odd...) Do I need to buy that ticket on-line now or can I wait until I get to Austria in a few weeks? That'd be 2 weeks prior to departure. I'd prefer to do it in person.
In my experience, European trains, particularly express trains, rarely fill up. Even then, there are plenty of unreserved seats, so if you buy your ticket two weeks in advance in Salzburg and reserve a seat for both trains, you should have no problem.
About the lack of frequent connections between Salzburg and Ljubljana: Remember that for decades, these cities were divided by the Iron Curtain. During that time, it was definitely NOT supposed to be easy to connect destinations on different sides of the divide. This is gradually being undone, but on high traffic routes first. Since Salzburg and Ljubljana are not as large as, say, Prague and Vienna, trains connecting them aren't as high a priority.
I would suggest getting them now on line if you can get a discount. Usually, there is a reduction of 50% if you buy ahead, but it should show on the on line booking. If there is no savings, then just wait, no sense committing $$ you don't need to until the time comes. I have rarely had a problem getting a set on a train. Have a great time.