Funny that people are so shocked about this... The myth that steel frames will last forever is being tested now that a significant number of cyclists have gone back to steel. BITD of all bikes being steel, frame failures were not uncommon.
There is another active thread right now about an almost identical failure of a Surly frame.
The difference is that if OP can prove they are the original owner, Trek will almost certainly give them a new one, while the Surly is out of warranty, and we have yet to hear what is going to happen with that..
I was concerned for OP that Trek would only have disc-only frames to replace it, but I just looked on their website and they still offer a canti-brake 520 frame!
Edit: The thread about the Surly.
https://www.bikeforums.net/bicycle-m...s-check-2.html
It is entirely possible that these bikes came off the same production line.