Someone was unknowingly given something poisonous from the murderer. That person entered the driver's cart and the drivers ate this poisonous thing and died. Marcia and the person opened the door to escape but then the murderer entered the room instead and killed Marcia.
Repeat this:
Only 3 person died in the driver's cart.
Only Marcia, the drivers, the murderer and you was ever inside the driver's cart.
No one in the driver's cart was killed with poison.