Emergency responders in Cincinnati know that trains full of crude oil have been rumbling through their city; they can see mile-long chains of black tank cars clacking across bridges over the Ohio River.

But they don't know enough to feel prepared for the kinds of fiery accidents that have occurred over the last 10 months after oil-train derailments. How many of the 100 trains that pass through residential neighborhoods and warehouse districts daily are carrying oil, for example? And when crude is carried, is it the kind that...