Einfach-Raus-Ticket

Absolutely. For easier navigation, go to www.bahn.com, enter WIEN to Salzburg, and enter a stopover in Melk for the amount of time you want to spend there, then check the "Local Transport Only" box. That will give you the eligible train routings. After 9 am week days, all day weekends.

As long as there are two of you, you will be able to use regional trains, but note that the Einfach-Raus-Ticket is not valid for a single traveler, only 2 -5 traveling together.

Austrian Rail will tell you exactly what trains you can use with the Ticket. Go to www.oebb.at. Change the language to English. Click on the red button, "Search connection". On the next page, put in from "Vienna", to "Salzburg Hbf. Select the date from the calendar and put in the time. Click on "Via" and, when offered the via, put in Melk and some time where is says "Duration ...". No need for the ':'. Put in e.g., 200 for 2 hrs. Go down to where is says "Products" and change it to Einfach-Raus.

Press search to get all the connections you can use with the Einfach-Raus-Ticket. Trains will be either 'R' for regional or 'REX" for regional express.

You can purchase the einfach-raus ticket at staffed stations the day of travel.

Per the Austrian Rail website,

"The Einfach-Raus-Ticket is available as Online-Ticket, at the ticket vending machine and at the Bahnhof in the City."

And yes you can purchase them for that price on the day of travel.

The Vienna to Melk bit is fine on regional trains as there is no other option. Getting from Melk to St. Polten is also regional. St. Polten to Salzburg is way too far for regional trains. Will take you forever. In St. Polten hop on the Westbahn train and buy cheap tickets while cruising at 200 km/hr.

Will take you forever.

The regional trains take time not for lack of speed. They take time because the stop at what appears to be every station. The regionals are a pleasant way to go for slow travelers who enjoy the journey as much as the destination.

Regional trains are not equipped to go 200 km/hr. I can promise you that the route between St. Polten and Salzburg is unexceptional and would be downright tedious on a regional train. Personally, I would much rather spend those hours in Salzburg than staring at train yards.

Thanks everyone for your replies. I think we have decided to change our itinerary and just do a day trip to Melk from Vienna. We are then going to head to Budapest for a few days before getting a fast direct train to Salzburg from Budapest. Thought about stopping at Melk from Budapest but it meant getting a really early train or getting in late to Salzburg as over the winter period we can only see Melk on a guided tour at 11am or 2pm.
Thanks again