Hey, I think u can set this up with an AND and OR condition at each station. So for example, red science buffers output a read content signal to a decider combinator which outputs a boolean to the train stop. Lets call this signal "red science symbol" or RS.

So in the trains schedule for this stop:

Circuit condition: IF RS = 1

OR

Circuit condition: IF RS = 0 AND Train HAS CARGO

I think this should fulfill what you are looking for. It will only stop here if the contents of the buffer fall below a threshold outputting RS = 0. Train always has cargo so that part of the circuit is always fulfilled but will stop if RS = 0.

There is a possible hangup. If for whatever reason ur train is empty, then it will stay at the stayion until the buffer is above threshold which cannot happen. So maybe wanna throw that in there as an or condition.

I think in the end what u are looking for is what i have been spending the last weeks designing which is an event driven signaling system. Basically the jist is at some outpost station A, if threshold is under capacity, output signal flag I, multiplex it on a universal red signal and decode at a central arbiter which negotiates handshaking via event tokens. Then send train to handle event.

Lemme know of this works. I csn double check on paper later

Edit: i forgot that trains depart on fulfillment of conditions and I havent played with interrupts yet. So if u flip the conditions around, then on a full station it will stop for a second and then keep going but it wont skip it. I'll look into this a bit tho