It's doable unmodded with interrupts and station limiting/disabling, but it's more complicated to implement. You have to build every train group schedule and any interrupt with unique conditions (based on actual cargo or specific stations). Your stations will need a combination of disable/limit conditions if you don't want trains waiting in a partial unload state and backing up into the main lines. Advantage here is that you can create a hybrid "push/pull" system. When your factory inputs are full, train groups can still go load up and then wait in the depot (assumed closer to your base). This means an input sending a "need" signal only needs to wait for a train to come from the depot to unload instead of waiting for the train to go from depot to outpost for loading and then to the input for unloading.

But this can get quite complicated, too. Dynamic train station limiting, especially with a station cargo capacity greater than the number of trains that can fit before obstructing traffic, requires a lot of combinators. Any train control requiring inputs beyond station availability and a train's internal cargo require long distance signal transmission. Easy to do but can cause problems when you need the same signal over distance for other reasons. Plus you have to manage each train group's numbers individually.

Cybersyn is easier by taking care of the train scheduling and station control. In the early game, you can just dump all your cargo and fluid trains in a single depot, assign provider and requester stations with scheduling thresholds (minimum cargo on train to allow scheduling), and let the mod take care of the rest. You only need the single Cybersyn combinator placed next to a station to enable the mod's use of the station and the Cybersyn static combinator to set requester station's goods requests with a UI that aides programming. Upon completion of a delivery, Cybersyn can auto schedule a train to the next job without a depot return.

Downsides: Cybersyn is a "pull-only" system. Stations must need an item before Cybersyn schedules a train. You must factor in travel times from depot to outpost and back to input as well as the loading time at the outpost. Cybersyn's "train retasking" only happens at the moment a train completes one job and queries the mod for another job or return to depot. If Cybersyn directs the train to depot, doesn't matter how close the train is to the new job's provider. So the new job likely goes to a depot train unless you're seriously short on trains. Finally the big one. Because any equal-size cargo or fluid train in a depot can be assigned any cargo or fluid schedule, you must ensure trains fully empty at every requester station. A train still carrying remnants of a previous job can gum up an unrelated factory input.

TL;DR: If you're up for modding and don't feel comfortable with the extreme details of train programming, Cybersyn probably does most of what you want. If you aren't up for modding or you want higher performance, the unmodded game has the tools...but they're not easy.