Maine Public Construction has started on Central Maine Power's corridor that is meant to carry hydroelectric power from Quebec into the New England regional grid, although the project still faces numerous legal and other challenges. Central Maine Power first proposed building a transmission line to Canada in 2017, and four years of controversy later, there are still hurdles ahead: legal challenges at the state and federal levels, a statewide ballot item set for November, unexpected technical issues to address. But on Thursday, a federal court lifted an injunction that barred tree-clearing in one disputed segment. And elsewhere on the corridor, trees are being cut and cleared, and poles are rising from the brush. Some observers say the $950 million project's gathering momentum could make it unstoppable.