Same day train ticket purchase

Pay more buy 100% refundable tickets in. advance.

You would search the German rail site bahn.de for Frankfurt to Salzburg, that site shows prices for 95-109 euros for a same day ticket

In my experience, there are "standard" fares for trains, not necessarily "demand" type fares, but most systems offer discounts based on early purchase. Two weeks out likely will not offer huge discounts over a few days out, or even day of, typically you need to be 30 days out or more. If you do some searching, you might find things like day or regional passes that can save money, but your trips are all long and International, so maybe not.

For Frankfurt to Salzburg, try DB Bahn (www.Bahn.com) the German rail site, OBB (www.oebb.at) for Austria, and www.mavcsoport.hu for Hungary. Typically you go to the country site for the originating city to find cost of tickets, the the German DB site works well to find schedules for most countries.

Deutsche Bahn for Frankfurt-Salzburg.

ÖBB website for Salzburg-Budapest and Budapest-Vienna.

I don't know whether you'd do any better on the Hungarian rail website.

If you don't mind paying a service charge (which I think is fairly modest), you could probably get all the tickets from trainline.com.

For general information about rail tickets, see Seat61.com.

If you look at fares for tomorrow, you'll probably be able to figure out the typical walk-up fare for each type of train (though they can vary by time of day in some cases, I think). Then you can look at fares for your actual travel dates and see how much you can potentially save by buying tickets now. In the current environment, it's hard to know whether there might still be some bargain tickets left just a day or so before you need them; I normally wouldn't expect to see that. You'll want to be sure you can get a seat on the train you want, and on any train with mandatory seat reservations, there's the at-least-theoretical possibility of a sell-out.

I would have a check on the rail websites for fourteen days' time as well - it's possible there may be refundable tickets still available that are more expensive than the ultra-cheap ones but still cheaper than for immediate travel.

Frankfurt-Salzburg:

Have you already committed to arriving at Frankfurt? To avoid a long train trip and high cost for last minute purchase or risk advance purchase, non-refundable tickets, I would book a flight to Munich Flughafen instead. Munich to Salzburg is about as fast by regional trains (Meridian) as it is by long distance train (EC or RJ), and you can purchase a Bayern-Ticket on arrival (no advance purchase needed) for the same price (€26 + €8 for persons 2-5) as in advance.

If you must travel from FRA, advance purchase Saver Fares (not Super-Saver Fares) are only a little more expensive than Super-Saver Fares and are refundable (with a penalty) up to the day before travel, so, if you can probably commit, look for them.