If I want more iron, I send a red iron signal to the circuit.
If I have more iron, I send a green iron signal to the circuit.
If a train can see a red and a green iron signal, it picks up the load at the green, and unloads at the red.
I can use several signals to add up (say 3 red iron means I ...

To recreate: Multiplayer, Host new game, PvP (next) (next) (play)
in tab 'balance options', set walking speed to 200 percent, (start round) (join a team)

Walk around: you walk at 200 percent. Walk to an alien, and die.
Respawn. You now walk at 100 percent.
I think this is a bug?

After some poking ...

I would like to see the following information added to the PvP commands on the wiki at https://wiki.factorio.com/index.php?title=Multiplayer#PvP :

Prerequisites:
- To play over the internet the server has to open router port 34197 and check 'Use P2P communications' while starting the server ...