A four-alarm fire Thursday in a Weymouth destroyed an apartment building that was under construction and affected commuter rail service in the area. The fire started at 3 a.m. at 71 Trotter Road. Flames started on the second and third floor and went through the roof. Firefighters had to battle the blaze from the outside because it was too dangerous to go inside the building. The fire chief said the condo complex was one month away from completion, but the sprinkler system was not operational. Finding a water supply was a challenge for firefighters.