Level on Quake 2 :-(?

I've just belatedly discovered what an ace game Quake 2 is. I'm stuck on the ammo depot and it's driving me nuts! I've extended the bridge and got through to that room. There is a small ramp leading up to a force field door and another door further into the room which opens at another point. There is also a door with a suspended carraige along from it which I suspect might smash through it? Have been all round this room and other rooms god knows how many times, and cant find a way forward. Please help!