Hey dude.
IMO trains need to be toned down in cargo to around 3000. They are too agile and quick to be that big.
Armoured transport to around 2800. As it can almost take on a Gunboat.
Whale needs to be the biggest ship as it gives up too much to be that small (3500 cargo).
I like the idea of a mammoth transport. Make its cruise speed 350 and thrust speed only 50. If it can not traverse through trade lanes either then 6000 cargo should be ok.( No guns, lots of Hull, like a battleship, lvl 6 shield.) You get hit? you pay for the damage.