Pour water over bran and let stand. In very large bowl, cream margarine, sugar and eggs. Add buttermilk and then bran mixture. Stir until blended. Stir flour, soda and add to above mixture. Mix well.
Fold in raisins last. Chill 1 day before baking. Fill tins 2/3 full and bake for 15-20 minutes at 275. makes 5 dozen muffins. The batter may be kept for 6 weeks in the refrigerator in a covered container.