[edit: replaced with a single complete example]
Would it be better for throughput to just carry the manifest with the train? No need to stop the train that way, and I think combinators are efficient enough, I think it'd be close enough to free. Here's a proof-of-concept: The split will shunt anything with less than 400 crude barrels onto the side path, put something else in the unconfigured loading chest to see it work, the output port on the signal propagators is the +0 accumulator connected to the next big pole. The extra combinators at merge entrances isolate the green-to-yellow transition, straight-line propagation works off just green-to-not-green so it's simpler.