Is it necessary to reserve a train from Munich to Salzburg, Austria on a Wednesday early afternoon? It appears that there are several trains.
There are plenty of trains Munich > Salzburg, roughly every 30 minutes at the time you want to travel. The chances of every afternoon train being entirely full on a weekday are miniscule to non-existent.
Use the official German railway website in English for times, details and cheaper advance online fares:
Taking advantage of cheaper advance online fares is imo the only reason you might want to buy tickets online/make a reservation in advance, unless you must catch a specific train at a specific time.
By the way, you won't need to join any 'long queues' for tickets at Munich Hauptbahnhof (Main Station) (the main station), even if they exist. Just use one of the numerous ticket machines. They have English language options, are simple to operate and at least some of them accept cash as well as cards.
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Necessary? Probably not. But you're always better off with a reservation...it'll put your mind at ease knowing you've got guaranteed seats and help you avoid wasting your time in long box office queues at the station.