Austria - Regional Train

You said "we" so I assume there are more than one (two?) of you. If so, you can use an Einfach Raus ticket (32€) for the Austrian portion of your travel. The Einfach Raus ticket is an all day pass for unlimited travel on regional trains in Austria for 2-5 people (note, it can not be used for a lone traveler). With that ticket you could travel all the way to Scharnitz, near the German border, but you would need some form of German Rail ticket to get across Bavaria to Reutte. Although Reutte is in the Austrian Tirol, it is serviced by the Ausserfernbahn, which goes from Garmisch-Partenkirchen to Kempten and is operated by German Rail. A German Rail ticket from Scharnitz to Reutte can be purchased online and costs 16,90€ person. Or, for only 26€ for two, you can purchase a Bayern-Ticket for the same travel. You could also get off the train in Garmisch-Partenkirchen and take a bus via Oberammergau to Füssen. That bus travel would be included with the Bayern-Ticket.

Thank you for the information. There will be 4 adults traveling plus an infant (8 months old). Do you know how much this pass will cost? What website can we look at to make a purchase of this pass?

You can read about the Einfach-Raus ticket here. The Bayern-Ticket would cost 34€ for four people. In Germany, the 8 month old would travel for free. In Austria the 8 month old is probably counted, but there is no extra cost for the fifth person.

You should really consider renting a car for that route. Tirol is best seen from behind the wheel, and you'll just see so much more. Austria is absolutely breathtaking in scenery.
And, it's very easy to negotiate the roads in the region. Some of the mountain roads are a little crooked, but they are in great condition.