Heat oven to 325°F. Grease an 8.5×4.5-inch loaf pan and line the bottom with parchment.
Bring a kettle of water to a boil for the water bath.
Whisk whole wheat flour, rye flour, cornmeal, baking soda, baking powder, and salt in a large bowl.
Stir buttermilk, molasses, and brown sugar in a separate bowl until smooth.
Pour wet ingredients into dry. Mix just until combined, then fold in raisins.
Scrape batter into the pan and smooth the top. Cover tightly with foil.
Set the pan in a roasting pan. Pour hot water around it to halfway up the sides.
Bake-steam until a tester comes out clean, about 60 to 70 minutes.
Remove foil and the pan from the bath. Cool 10 minutes, then turn out and cool 30 minutes.
Slice warm or at room temperature. Serve with butter or cream cheese.