The split won't actually raise capital for the deal, but will help to payoff shareholders of the railroad. According to http://www.marketwatch.com/story/berksh

The split won't actually raise capital for the deal, but will help to payoff shareholders of the railroad. According to http://www.marketwatch.com/story/berkshire-approves-50-for-1-class-b-stock-split-2009-11-03 http://www.marketwatch.com/story/berkshire-approves-50-for-1... the split is to "accommodate holders of smaller amounts of BNSF shares who opt for a share exchange rather than a cash payment." So, some shareholders of Burlington will opt for shares instead of cash, and since you can’t own a fraction of a share, they have to split the stock to accommodate them.