If all goes well for Irving on proposed plans to build a Verizon-anchored, mixed-use development along the city's relatively new DART rail line, Mayor Beth Van Duyne said the city's $1 billion investment in the transit line will have paid off in a big way. "We are in discussions right now for a large corporate project and a mixed-use project with retail and residential, which was interest sparked by the DART line," Van Duyne told the Dallas Business Journal."The station hasn't been built yet, but…