transporters
The transporters standing still right now because this one is in their way.
Answered exactly rather than guessed: a blocked transporter records the zone or the link it is waiting for, so being in the way is a fact about what this transporter holds, not an inference from where it happens to be.
What it is for is a policy that has to decide whether a stopped vehicle is worth moving. A vehicle broken down on a spur nobody uses can be repaired where it stands; the same vehicle across a main aisle cannot, and this is the difference between the two.