Dropoff stations get overfilled if configured with true maximum capacity

[saxbophone]

Temporary workaround: set station capacity M to be the true maximum capacity minus the size of a train-load (W), as in $M = truecapacity - W$

This workaround has been thoroughly tested and shown to work. With the workaround applied, stations do not get overfilled to the point of jamming, they only get filled up in some cases to more than $M$ but always less than $truecapacity$.

Without a more reliable fix available, the simplest fix will be to amend the design slightly to make this subtraction within the circuit itself, so that users can set $M$ to $truecapacity$, with the circuit subtracting $W$ from this number to provide the capacity actually used in the internal calculations.