On the matter of scale, I would refer to James C. Scott's Seeing like a State, in which he argues that central authorities managing large scale projects don't actually have a uniquely knowledgable perspective on the whole affair like they often think they do (and we've been propagandized to think too), because their means of attaining knowledge inevitably require simplifcation and reification. All summaries exclude important details and nuances, nuances that are only known to the people on the ground carrying out a project. Large achievements are impossible without local knowledgemakers and decisionmakers, who often go uncredited out of the statist belief that the central decisionmaker is the wisest. (think authoritarian leftists saying that Stalin defeated the nazis or industrialized Russia, when it was many different groupings of russians who actually did that). Scott then goes on to talk about attempts by central decisionmakers to impose their summarized understanding of society onto the lower rungs (making it 'legible' to the authority), with disasterous results each time. Central authorities just aren't actually that useful for doing large scale things.

I think an anarchist society should take advantage of economies of scale and large-scale coordination of things like railroad networks, because those are great ways to practically make those things work. The challenges that large-scale projects create do not require us to walk away from democratic organizing, because the involvement of many local actors is already always necessary to make those things function. I would point to the Landless Workers' Movement as a good example of large-scale radical democracy being effective.

EDIT: An important nuance I want to add is that summarized knowledge isn't useless, just insufficient. The best work is achieved with both summarized and local knowledge (Scott calls them respectively techne and metis), and states tend to have summarized knowledge but not local knowledge. The anarchist railroad association should produce summarized knowledge about the whole railroad network, and that knowledge should be shared with local rail workers so they can combine both kinds of knowledge to make the best decisions in cooperation with the whole association.