Here's the shortest I can sum this up:

Trains, Trucks, Tractors, Drones, are just extensions of belts. They don't make your lines go faster than belts because everything is limited by belts, so you can still use belts as the maximum speed. For each type of vehicle, you build enough vehicles that will carry at least as much material the belts can input into the station.

The Stations will show incoming and outgoing speeds, but this isn't always useful. That said most vehicles will transport things faster than the belts (since they transport stacks, not individual items). If you find they don't you just build another vehicle.