Hi,
in this situation I would have expected truck B to choose the free freight yard because A is blocking the first one.
How would this be done correctly?
(http://i.imgur.com/cHhNZ.jpg)
It seems you built that sign in wrong way - change direction of that sign.
In this case choose sign works in direction of departures from station on screenshot.
Yes, and build it one tile earlier, e.g. before the road split.