Mattapan is an excellent place for photos, but i would definately bring a friend along. You can argue to death the safety of the station, but you cannot tell me ashmont is safe when its the most popular station on the news. and dorchester is undoubtably a bad area, and i wouldnt want to be subject to the potential dangers on the mattapan-ashmont line.
also, may I recomend Charles/MGH. It is a very popular spot, you can see the trains descend off of the longfellow bridge into the station, and out of the tunnel from the outbound platform.
And the Snow plow and wire car are still there.