PHL controller described an aircraft/vehicle conflict when an unauthorized airport vehicle entered the departure runway with an aircraft on takeoff roll; the departing aircraft overflew the truck and an arrival was issued a go around.

Narrative

I was conducting OJT (on job training) position when a CRJ1 was cleared for take-off. The aircraft started his takeoff roll and after about a thousand feet the AMASS (Airport Movement Area Safety System) alerted and announced that the runway was occupied. I looked at the display and saw a blue splat at the opposite end of the runway. I initially thought it was false target until it started moving toward the aircraft taking off. The CRJ1 was more than halfway down the runway and I could not give any instructions for fear that the aircraft would not have enough speed for lift off or too much speed to stop before the truck. The CRJ1 was able to takeoff and flew over the truck. To my surprise; the truck continued down the runway and I was forced to send an arrival air carrier around. I tried to call the truck but I had no call sign because the Airport Operator refuses to put transponders in their trucks. The truck finally exited the runway at Taxiway Yankee. If the Airport Operator had transponders in all of its vehicles I would have seen the vehicle before it entered the runway and a call sign to use.