In 1972, The O'Jays released their hit song Love Train. A music video was recorded one year later in 1973. The music video was considered lost, as there were no videos of it on YouTube. (There are videos of The O'Jays performing it live, but those have nothing to do with the music video itself.)
On August 19, 2016, TheSponge231 found the music video.
According to Wikipedia, the music video shows a group of people forming a chain near a railroad station, while at the same time, some railroad cars are shown in motion.
Throughout the video, more people join the chain, which they call the "Love Train".
It was most likely filmed around the Northeast Corridor, as Long Island Rail Road MP75 cars appear throughout. (in which the words "LONG ISLAND" can be seen), as well at Amtrak cars and other rail cars. Not much else is known about it, other than the fact it was recorded in 1973.
UPDATE 8/19/16: TheSponge231 recently discovered a higher pitched version of the music video, and can be viewed here: https://www.youtube.com/watch?v=LXlsJKP_FWI
UPDATE 7/21/17: shadowkoishi93, the uploader of the music video that TheSponge231 found, has re-uploaded the music video at the correct pitch. It can be viewed here: https://www.youtube.com/watch?v=PCUdj0Wzf1k