Photo by solson After Sen. Barbara Mikulski (D-Md.) went off on WMATA for lax safety standards and endless excuses at a Senate subcommittee hearing on Thursday (C-SPAN’s got the video here, Mikulski shows up around the 40-minute mark), suddenly Metro Board Chairman Jim Graham is talking about firing some people. The Washington Post reports this morning that Graham claims that “very substantial management changes at Metro” were already in the works before Mikulski spoke…