loading a waggon properly?

[brunzenstein]

I want to send a waggon with e.g. a dedicated set and quantity of material (solar panels, accu, wires e.g.) to a solar outpost - how to get the robots to prepare just the needed quantity of the desired items and load that to a waggon?

[Bushdoctor]

If you want to load a train with a predetermined set of items, you can just lock the boxes inside the wagon to only accept specific items.

If you meant that you want a train to only load the EXACT amount of items that you need to complete a building job, that is going to be very difficult.
It would come close to a self-building base. If you're curious HOW difficult that's going to be, then check out this video: https://youtu.be/xF--1XdcOeM

[Engimage]

You can do it by logistic slots in the logistic chest and then moving all of its contents into the train.
Be aware that you should not be offloading items into the train while they are delivered by bots if you want the exact amount. Also when you start loading a train you should replace requester chest by storage chest to prevent redelivery. This is the way to do a one-time load.

As an alternative if you need it on a regular basis you might reserve wagon slots to certain items and put requester chests near the station so the system will try to load all the available slots in the wagon. This is the most used way to do it. If you need really lots of stuff you might dedicate whole wagons to a certain item without hassling with slot reservations but that might end up not very well as train might change directions in the process and come back to loading station with head and tail swapped effectively messing up its contents.

You might also make use of circuit network. Train station can output items in the train cargo. So if you have dedicated requester chests for different types of items near the wagon you can switch respective inserters on and off based on the difference in a constant combinator (in which you set up your request) and train cargo.

[Yoyobuae]

Did this for someone else asking similar question:
https://gfycat.com/RareSpottedDogwoodclubgall

Requesters chest > Inserter > belt > Inserter with read hand(hold) and item limit > Steel chest > Inserter > Cargo Wagon

Train is prevented from entering the station before the steel chests have the correct contents and once the train enters the steel chests are not refilled anymore until train leaves again.