on Friday may 26, around 1930 local time in BOS, we were waiting for our friends to depart to LHR. we had a great observation spot, on the top level of central praking giving us clear views of the whole airport. anyhow, a follow mecar with lights flashing and all was leading a delta md90. ok, thats normal. but then, it crossed rwys 22R/4L and 22L/4R. these were the only active runways at the time. they went all the way out to the end of rwy 33L were the follow me car stayed on the rwy while the delta held short. they remained in this position close to 40 minutes. any idea what it could be? testing?