Online sports betting giant BetMGM has signed an agreement with NJ Transit to get its message out to riders and sports fans, becoming the only corporate sponsor of a rail line in the nation. NJ Transit’s rail line to MetLife Stadium and the Meadowlands sports complex now carries the name of the Jersey City-based sports betting company under a three-year, $3 million deal announced in the Secaucus Junction rail station Wednesday. Signs were already up in the station with the new name BetMGM Meadowlands Rail Line, in addition to ads and massive banners for the company, including some ads on track…