Sure, and the warning signal doesn't travel at the full vacuum speed of light, either: part or all of its path goes through electronics, and electrical signals

Sure, and the warning signal doesn't travel at the full vacuum speed of light, either: part or all of its path goes through electronics, and electrical signals travel only something like 0.5c. But this is a really short article aiming to give readers the gist of how the system works. Referring to "the speed of sound" without specifying the material is still probably within an order of magnitude or so of being accurate, and I don't know how valuable greater precision would be in this context. (In any case, I'll wager that the number of people who 1) have a solid, intuitive understanding of the 350m/s speed of sound in air, but also 2) don't understand that earthquake wave speeds in rock are also essentially sound waves but travel significantly faster, is pretty low.)