State regulators have awarded the last remaining permit needed for construction to start on a $12.7 million layover facility for Amtrak trains in Brunswick. The Maine Department of Environmental Protection on Tuesday approved the stormwater management plan submitted by the Northern New England Passenger Rail Authority. The permit was the project’s final regulatory hurdle. Regulators concluded that existing berms, ditches and adjacent railroad tracks would prevent significant amounts of storm water runoff from flowing toward abutters, and that the groundwater at the site generally flows toward a nearby stream and not to an adjacent neighborhood. There has been intense opposition to the facility since it was first proposed in 2012 when the Downeaster service was extended from Portland to Brunswick.