Let me explain please in a few words
1. valve is "Cough" and they reliaved themselves of 10000000000000000 line encoding of the "SOURCE" engine and well read 2
2. All the people have the weigt "1" and all the bullets have the weight 1-slash (excluding nades they are 1)
3. all the objects in Valve are pinned down with Hammer (literaly) to the floor, and that makes them not move and fly (stones, houses, and crap whick makes the map look nice) on the other hand u have crates and "THE CART" since the cart is not a Phys_OBJ not it has a skeleton (player/Creature based) it has no Weight !!!
All that makes it held down is a Friction based Invisible rail system, which pushes it forward like an Horisontal elevator, so ...... Lame "Cough, Sneeze, Momble, Cough, Cough