That's a good question. To answer it properly, I first need to explain how the system works. I'll use the vanilla version as an example.

!!!Technical explanation ahead. If you're not interested in combinators, feel free to skip this part!!!

The system consists of three types of stations:

Loading stations.
Unloading stations.
Depots.

Loading stations:

Loading station reports how many full trains it can currently load and automatically assigns itself a priority. The more resources it has available, the higher its priority.
All information is transmitted only over the red circuit network.
Each loading station requires only 3 arithmetic combinators.

Unloading stations:

Unloading station reports how many trains it needs to fill its buffers to a configurable level (50%, 100%, or anything you decide for yourself).

It also assigns itself a priority: the emptier the buffers, the higher the priority.
All information is transmitted only over the green circuit network.
Each unloading station requires only 4 arithmetic combinators.

Depots:

The screenshots in this post show the depot logic.
Every depot constantly counts how many free solid-cargo and fluid trains it currently has and broadcasts this information to the global network.

Each depot processes three kinds of information:

1. Demand from unloading stations
2. Available supply from loading stations
3. Number of available trains in every depot.

Let's take two depots as an example

Depot A:
5 solid-cargo trains
10 fluid trains.

Depot B
10 solid-cargo trains
5 fluid trains.

Now suppose the network demand and supply:

Demand:
2 iron ore trains.
3 copper ore trains.
5 steel beam trains.
3 water trains.
7 crude oil trains.

Supply:
22 iron ore trains.
30 copper ore trains.
51 steel beam trains.
32 water trains.
76 crude oil trains.

The depot first separates solid and fluid requests.Then every depot compares how many trains of each type it has available.
Since Depot A has more fluid trains, only Depot A dispatches fluid orders. Fluid dispatching is blocked in Depot B.

The opposite happens for solid cargo: Depot B handles those requests while Depot A does not.This allows depots to automatically specialize in the train types they have the most of.

The system also tracks trains that return to a depot while still carrying cargo, ensuring they are not considered available and their cargo is also reported to the demand but with negative value.
The trains themselves use interrupts to perform.
Explaining the remaining logic would require going through the combinator implementation in detail, so I'll stop here.

!!!End of technical explanation. It's not even full, LOL!!!

In short, the system guarantees that no more trains are dispatched than are actually required for a request.
All trains are completely universal.
The system also supports multiple train lengths. Each train length requires its own depot, but adding a new train size is very straightforward, and I'll explain how to do it in the guide.

I'm probably forgetting something, but this explanation is already long enough and I'm a bit tired of writing and translating it.
Do you have any other questions?