Cargo Only Airlines

Possible? I have seen a few, but can never understand how people operate them, do they try follow the same departure wave system or what? 

Cargo is more difficult, because it doesn't follow the usually pax pattern. LHR isn't necessarily a good cargo hub. But CGN is. Or LUX. Also, many pax long haul flights carry cargo very cheap, which makes it hard to compete.

However cargo has “less” players. You can easily find a game world with only 1-3 dedicated freight airlines. And if you operate ATR’s to minor airports, you are likely to succeed. I have had a cargo company with more than a 1000 planes - but it’s a lot of work, and yes working in waves is just as good as with pax.