In - of all places - the Museum of East Anglian Life, Stowmarket, Suffolk - I saw this photo on display. Its source wasn't given, beyond 1879, India. (It was part of a hall displaying ploughs and other agricultural machinery.)
Both Sukumar Raman and Jacob Cheeran, Asian elephant experts I talked to, said they'd heard of, or seen photos of, ploughing elephants. Cheeran said he'd seen early 20th century photos of ploughing elephants in Burma. He said the biggest problem with ploughing elephants would be the gears for the plough, it being such a big animal it would be hard to set up the gears. (Most contemporary ploughs, including those pulled by bullocks in India, are assumed to have gears these days.)
Pliny the Elder in his Natural Histories talks about "nothi", "bastard elephants" used for ploughing in India, assumed to have been smaller elephants. But as you can see from the engraving, it's a massive, adult male tusker pulling an absolutely huge plough. It's quite possible, of course, that the drawing's not accurate. The ploughmen (ploughmen-mahouts?) don't look particularly like farmers to me, and the elephant, with its straight tusks, isn't particularly well rendered.
