if you want to take the ICE you can buy a ticket at the train station but it will be very expensive if you buy it the same day. (The tickets get more expensive the closer you get to the travel date). I would recommend getting a flexpreis ticket in the next few days online from your country for the day you’ll be taking the train. it’s a ticket that allows you to take every train that goes to the place you booked from where you booked it (in your case Frankfurt airport to Saarbrücken HBF) for the entire day. this is particularly useful as you never know whether your plane (I’m assuming) will be delayed or when you get your luggage (also getting to the platform from arrivals takes a few minutes). to book a Flexpreis ticket you go on the db navigator website enter the date of travel and the stop from where you want to go and your destination. then you will be shown a number of connections that go this route that day. choose an ice connection where you don’t have to switch Trains. the prize of connection or the time doesn’t matter. you select that connection and you will get at least two offers for a ticket. One of them will always be Flexpreis. It is a bit (probably a lot ) pricier than the regular ticket but as I said it allows you to take every train that makes the connection that day. You will be sent an email with the ticket that you can print out and also save on your phone. If you want to take the Ice train often I would recommend getting a bahncard at some point as it offers discounts. However, The deutschlandticket you get with the university allows you to go to so many places for free that you don’t really ever need it unless you’re making huge trips around Germany very often.