Yet another 4chan meme. On Fridays, the 4chan /b/ board used to get flooded with ‘Furry’ hentai posts – in honour of Furry Friday. These images feature anthropomorphic animals in a sexually explicit way. To counter this ‘scourge’, the guys at /b/ created F40PH Friday.
The F40PH is – well, it’s a train.
According to Wikipedia, the General Motors Electro-Motive Division model F40PH is a 3000 horsepower B-B diesel-electric locomotive built from 1976 onwards. Supposedly, it's quite a train - equipped with a turbocharged 16 cylinder, two-stroke, water-cooled "Vee" diesel engine.
Anytime anyone started a ‘Furry Friday’ thread, it would be flooded with pictures of the great F40PH. Often 4channers use a directory dumper to mass-dump pictures of the train into the thread, taking it over.
Here's a video featuring some of the F40PH pictures used on F40ph Friday: