The Deutsche Bahn website was used to calculate all the below journey options, with 28th Oct 2019 as the (fairly arbitrarily chosen) notional day of travel.
So, what are your options...?
1. Anywhere in GermanyOkay, this is a slight exaggeration. The shortest time I can see to get to Westerland, on the island of Sylt, is 12:25. And Ostseebad Binz, on the island of Rügen, takes at least 12:59, with the quickest route to Stralsund coming in at 12:06. Hopefully you won't begrudge me the extra few minutes, though, especially since none of these are particularly common destinations for academic business trips.
Hamburg, on the other hand, can be done in 8:13 with only one change. Berlin can be done in 8:36, and Köln in a mere 5:01. This may not be particularly surprising to anyone, though, so let's move on.
2. Anywhere in Austria, Belgium, Liechtenstein, Luxembourg, Monaco, or SwitzerlandAustria is not as long as it looks. Graz can be got to in just over 10 hours, Klagenfurt in just over 9, Vienna in less than 9. The longest trip I've been able to find is to Jennersdorf, on the border with Hungary, and apparently a fairly unremarkable place. Still doable in under 12 hours.
As for Belgium, a trip to De Panne, on the North Sea coast near the French border, will be over in the blink of an eye (9:59). The pretty little town of Couvin, at the end of a branch line in Wallonia near Namur, will take you 10:26. More usefully, you can reach Brussels within seven hours.
Liechtenstein is obvious, as is Luxembourg (just over 6 hours). Almost everywhere in Switzerland is accessible within 5 hours; if you want to train it to Brusio with its spiral viaduct, high in the Alps on the Bernina railway, you'd better leave 6:20.
|The Brusio spiral viaduct (CC-BY 3.0, by Kabelleger)|
3. Most places in Czechia or the NetherlandsOstrava, the Czech town that's the furthest from Konstanz (near the border with Poland), will push you over the limit at 12:20, as will Olomouc at 12:09 (just). But Prague can be reliably got to in a piffling 9:32, and Brno in 10:38. Meanwhile, Amsterdam is a comfortable 8:35, and Groningen or Rotterdam can be reached in under 10. If you want to go right to the North Sea coast on a branch line, brace yourself for a longer journey, though.
4. Denmark, Hungary, Poland, Slovakia, SloveniaBratislava is only 10:13, and Ljubljana a mere 10:24! As for Denmark, Hungary and Poland, admittedly, you probably won't get to anywhere useful within these countries in 12 hours. But Budapest, for instance, is only 12:09, Poznan is only 12:16, and places like Aarhus, Copenhagen, Warsaw and Wroclaw are doable as part of a longer day. Zagreb, in Croatia, is only just out of reach at 12:44, and there's also a sleeper that goes there from Zurich.
5. Northern and Central Italy, and almost all of FranceAt 5:41, Milan is embarrassingly easy to reach. From there you can travel onwards to lots of other major cities within the 12-hour limit: Florence at 7:37, Rimini 8:29, Venice 8:31, Rome 9:14, Trieste 10:30, Naples 10:47. Only the far south and Sicily take longer.
France is astonishingly accessible. With Paris less than five and a half hours away, you can be there in time for a leisurely lunch. Connections via Paris, Dijon and Strasbourg will also get you to places like Lille (7:17), Marseille (7:42), Bordeaux (8:16), Nantes (9:07), Toulouse (11:05), Bayonne (11:07), and even Brest at the very tip of Brittany (an incredible 10:08, if you can leg it across Paris fast enough). You'll only struggle with getting somewhere in France within 12 hours if it's at the far end of a tiny branch line.
If you are a fan of micro-states, Vatican City (via Rome), San Marino (via Rimini) and Monaco (10:15) are all within your grasp. And Barcelona is SO DAMN CLOSE to being under the limit (12:21), putting you not that far from Andorra (okay that's a stretch but work with me here). Another Spanish destination that's easily doable within a day is Irun, in the Basque country, at a fairly comfortable 12:36. And if you stay on that last train you'll end up in Lisbon in the morning.
6. The UKLondon is reachable in 8:59 by Eurostar via Paris, if the patron saint of connections extends his blessing to you. More usually it'll be about 10 (you need to hang around a bit in Paris). The Eurostar will also allow you to travel on to Cambridge (10:23), Oxford (11:04), Birmingham (11:17), York (11:29), Sheffield (11:55), and even Manchester (11:56), just. Within a slightly longer day you can also get to the capitals of Wales (Cardiff, 12:36) and Scotland (Edinburgh, 14:22), as well as Leeds (12:07), Newcastle (12:44) or Lancaster (12:46).
So that's not a bad range of places for half the time it takes for the Earth to rotate on its axis. Here's a map giving an overview:
With options like this, why would you fly short-haul to any of these places?