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I saw it once! Kind of.
After Gion Matsuri in Kyoto last summer, I was catching a train back to Nagoya and noticed lots of people on the opposite platform. I thought nothing more of it, but when I boarded my train, the windows were... yellowed out. I figured out what was happening fairly quickly, but I probably wouldn't have had time to get back out and take a good picture.
Hopefully one will be preserved either at JR West's museum in Kyoto, or at JR Tokai's SC Maglev Railway Park in Nagoya next to its predecessor.
ドクターイエロー。