The Nikkei stock index topped the 50,000 line for the first time early Monday, tracking Wall Street gains after key U.S. stock indexes ended at record highs late last week. In the first 15 minutes of trading, the 225-issue Nikkei Stock Average rose 948.64 points, or 1.92 percent, from Friday to 50,248.29. The broader Topix index was up 51.66 points, or 1.58 percent, at 3,321.11.