The Flixbus I took from Berlin to Hamburg last month showed up about fifteen minutes late. My seat didn’t recline. The wifi was spotty. The bathroom, reasonably clean. In other words, at 18 euros, the three-hour ride was just fine—a step up from what intercity buses have historically offered, but nothing revolutionary.

And yet Flixbus is the hottest thing in Europe’s long-haul bus business: It boasts 200,000 cheap, daily connections to more than 1,200 destinations in 26 countries, carrying 30 mil…