"Cattle class" is a kinda faux-pejorative term for economy/coach/whatever the cheapest cabin is in some mode of transport.
People sometimes aren't just stacked, they're squashed.
"Don't sit with the cattle" is (IMHO) pretty harsh, and unnecessary.
And, for the record, cattle class seats are (or were) about the size of first-class airline seats, in the days before the bed capsules. It's not a cabin but it might be the least-cramped train seating you will ever encounter.
This is true, they're FAR better than standard airline seats. On the other hand, if you're going NY to SF you're going to be sitting in them for 3 days, so they better be comfortable.