Let’s say 10 tanker cars so 350,000 gallons or 560,000kg are spread down a water channel at the side of a street at 15mph. At 4 minutes their already covering 1 mile which easily takes them out of most towns making longer durations largely meaningless. If the channel is V shaped with a depth of 2 feet and a width of 10 feet. That only adds up to ~4,5000 square meters, and by your calculations most of it should still be there.

It doesn’t last as long if it’s on fire, but there’s going to be an air gap of unmixed natural gas over a likely several feel thick channel of LNG. So even then it’s going to take a while to burn off meanwhile the wave of fire is destroying everything around it.

In a real scenario, the cars are going to be spread out over an area, their ruptures will be relatively small orifices that the fluid must flow through, it's going to spill out rapidly in all directions from that point, any area where a train is passing through is going to be decently wide and flat, worst case scenario probably a 50 foot wide street.

You can look up pictures of derailed tanker cars, for example [0], and see the area where the liquid spills over. By the time the contents of a tanker car have spread over an area of 1 acre, the fluid is only 1in deep, even if none of it vaporizes it's limited in how far it can meaningfully spread. The spill of a cryogenic fluid will by necessity cover less area. As you can see, the spill of 322,000 gallons of flammable liquid affects an area comparable to that threatened by the derailed train cars themselves. That fire lasted 36 hours, wouldn't it be great if all the fuel evaporated in 36 minutes?

[0] https://www.progressiverailroading.com/federal_legislation_r...