Environmentalists returned to NJ Transit roughly 15 months after they successfully lobbied the agency to build a back-up power plant using solar and renewable power, to criticize it for allowing a natural gas power plant as an option. The 140 megawatt generator in question is part of the larger NJ Transitgrid Power System, a $577 million project that would provide emergency electricity to Amtrak’s Northeast Corridor, parts of NJ Transit’s Morris and Essex lines and that’s used to run Hudson-Bergen light rail in case of a power outage. The project is partly funded with $410 million in federal H…