Place brisket and juices from bag in a large Dutch oven with cold water to cover; insert meat thermometer into thickest part of brisket, if desired. Bring to a boil. Reduce heat; cover and simmer 3 1/2 hours or until brisket is tender or meat thermometer registers 180°. Remove brisket from water; let stand 30 minutes. Cut into thin slices diagonally across the grain.
To Make Sandwiches: Place corned beef on half of Dark Rye Bread slices; top each with a tomato slice, a lettuce leaf, a cheese slice, and a teaspoon each of mayonnaise, mustard, and horseradish. Cover with remaining bread slices.