U.S. stocks were mixed by midday Tuesday with earnings from JPMorgan (JPM) pushing the Dow higher while a sell-off among railroad companies hurt the S&P 500. Norfolk Southern (NSC) reduced its first-quarter profit guidance to $1 a share, short analysts' estimates. The company said a decline in export volume of coal had hurt its bottom line. Industry peers Union Pacific (UNP) and Kansas City Southern (KSU) were also lower. Online real estate service Zillow (Z) pressured the Nasdaq after issuing full-year revenue below expectations. Oil extended gains into a fourth straight session, climbing above $53 a barrel.