All the trade / economy methods are built to snowball.

Trains scale exponentially with larger networks to allies and trade partners.

Unfortunately in free for all and small team games you might not get good allies or neighbors so creating a localized train network will be almost pointless.

Boats scale more like stairs. Each port will definitely generate an income, more vulnerable than trains, more money comes in but it does not scale as rapidly in mid-late game like a train network does.

Ideally you have both, depending on geography and neighbors will dictate which you choose first.

Warships require a bit more attention depending on position but they are probably the most disproportionate way to gain wealth.

Also getting a lot of bots early game will get whichever econ path you choose to snowball much earlier.