PARIS — It was 5:45 p.m., a normal Friday afternoon on the sleek high-speed train that takes high-level European diplomats, businesspeople, tourists and ordinary citizens between Amsterdam, Brussels and Paris.

Less than an hour away from Paris, a French passenger got up from his seat to use the toilets at the back of the carriage. Suddenly, in front of him rose a slightly built man. Across the man’s chest, in a sling, was an automatic rifle of the kind favored by jihadists the world over: an AK-47.

The passenger threw himself on the man. The gun went off, once, twice, several times. Glass shattered. A bullet hit a passenger.

The man with the gun kept going down the carriage, holding his AK-47 and a Luger pistol. In a pocket was a sharp blade capable of inflicting grievous harm. He had at least nine clips of ammunition, enough for serious carnage.