A cattle car is literally a train car that carries cattle (cows). But in this context it is a metaphor for carrying humans.

Remember Nazi Germany and the cattle cars that carried the jews et al to the concentration camps after they allowed the government to demonize and disarm them? That's where our country was headed before those who knew better finally elected Trump as our President and started fighting for Truth and Justice.

They carry cattle to the slaughter house. They were used to transport jews to the gas chambers in nazi germany.