NEW YORK (AP) â¿¿ Shares of railroad companies tumbled Wednesday on concerns about lower coal shipments. Coal is a major category for railroads, and a significant drop in coal shipments could hurt railroad company profits. On Wednesday, Alpha Natural Resources Inc. said it would ship less coal this year than it had previously expected because of declining demand from Asia and production shortfalls at some U.S. mines. Another company, Walter Energy Inc., also cut its coal production forecast for the second half of the year, and shares of coal companies declined broadly. Railroads and coal companies posted some of the deepest declines in the stock market Wednesday. In midday trading, railroad CSX Corp. dropped $1.13, or 6 percent, to $19.10. Over the past 52 weeks, its shares have traded between $17.98 and $27.06, and are down 26 percent in the past three months. Norfolk Southern Corp. shares tumbled $3.91, or 5.8 percent, to $63.63; Union Pacific Corp. lost $1.95, or 1.7 percent, to $86.66; and Genesee & Wyoming Inc. fell 81 cents to $48.69.