Okay I was stumped about how this works because it's not explained, as far as I can tell. But I guess the sensor array has its long axis perpendicular to the direction the train is traveling.
Absolutely fascinating stuff! Thank you so much for adding detailed explanations of the math involved and your process. Always wondered how it worked but never bothered to look it up until today. Reading your page pushed it beyond idle curiosity for me. Thanks for that. And thanks also to HN for always surfacing truly interesting reading material on a daily basis!