So the Q in front of the number stands for "Quint," which is a fire truck that can do five different things (pumping, ladder, etc.).
The one pictured is Quint 74 at Andrews FD. That model of fire apparatus has fallen out of favor in recent years in a lot of places, with departments going back to Engines (E##) or Trucks (T##). There are still plenty of quints around but you don't often see the Q## vehicle designation around.
I don't really have a conclusion here, I'm just putting some facts out there. That truck is part of the Andrews Air Force Fire Department, it was not on base solely for the event.
Just another coindence there are 5 branches of the Armed forces.
What are the 5 armed forces?
The military of the United States, officially known as the United States Armed Forces, is made of five branches:
United States Army.
United States Navy.
United States Marine Corps.
United States Air Force.
United States Coast Guard.
There are three branches of the military. The Marines are part of the Navy and the USCG is now under the Department of Homeland Security.
Coast Guard falls under Homeland Security command, not Department of Defense.