The Reuben sandwich is an American hot sandwich composed of corned beef, Swiss cheese, sauerkraut, and Russian dressing, grilled between slices of rye bread. Several variants exist.

What is the meat in a reuben sandwich? Let's have some stream of consciousness first.
The relevant sentence in the passage is: The Reuben sandwich is an American hot sandwich composed of corned beef, Swiss cheese, sauerkraut, and Russian dressing, grilled between slices of rye bread. So, the answer is corned beef.