The key stock averages notched modest gains Wednesday, and the popular Dow Jones industrials marked an all-time high. But a closer look shows the session was much more mixed. The S&P 500 rallied 0.4%, stamping out Tuesday’s small decline. NYSE volume edged up. The large-cap benchmark is just a few points away from painting another all-time record. The Dow transports rose 0.6%, reflecting high demand for airline, shipping and trucking stocks.