Honestly, it's a waste of time to try and collect every single slug you see. You really only need a few.

IMHO, they're only really useful for speeding up resource extraction from mines. For everything else, it's better to just build more machines than to overclock the existing machines. Better to build two machines, rather than overclock one for double production. So the slugs are only useful for machines that you can't simply build more of, either because there are limited places on the map you can put one (like miners) or because you've painted yourself into a corner with your factory design and don't have room to add more.

For low volume cargo, it's usually not a problem. For example, I manufacture computers at the Desert Advanced Manufacturing Node in the rocky desert, and I ship them by train along with five other products also made at the D.A.M.N. to the PRoject Assembly Tower to be made into adaptive control units and to the Super Heavy Industry Terminal to be made into Radio Control Units. In both cases they just get belted straight to the machines that take them because those machines won't realistically consume an entire train load before the train returns.

For high volume cargo, I handle it like this: I make plastic at the West Heavy Oil Resource Extractor, 60 refineries worth. Each row of 10 refineries is belted into an industrial storage container with one MK4 belt, and then that storage container is belted into a single freight platform with two MK4 belts. The same is done at the D.A.M.N. to unload the train there for manufacturing computers, supercomputers and such. The same is done at the Aluminum Swamp Site to handle the massive amounts of silica the plant consumes as well as the huge number of aluminum casings produced, which are shipped to the S.H.I.T to be made into...everything made there, directly or indirectly, from fused frames to nuclear pasta.

I call that technique a "buffer box." The storage container won't block when the station is loading or unloading a train, and it can be fill the platform or be filled from the platform twice as fast as the belt from/to the factory. I then plan each freight platform as if it can carry the same throughput as the belt that feeds it.

I will sometimes split the box/platform connections, connect Box 1 to platform 1 and 2, Box 2 to 2 and 3, and so on, and then box 6 to platform 6 and 1. Sometimes at the load end one end of the train or one specific box or something will end up feeding faster than others so that's a way to balance it, so I don't end up with 5/6ths of my A.S.S. totally backed up with the remaining 6th struggles to meet demand.

A quirk of industrial containers: They sort of work as mergers and splitters. They don't guarantee proportional bandwidth the way actual mergers and splitters do; you might see one of the two belts going into a container feeding while the other is backed up, or one of the two coming out of a container full while the other is empty, but since we're connecting the side with 2 belts to the same platform it doesn't functionally matter in this use case.

What’s random about them? In my factory they seem extremely reliable.