The current coupling algorithm bugs around when trying to couple wagons
with a very short wagon_span.
This can be tested using the minecarts from rubberduck's moretrains.
Should be solved by 1f3a4c3b and 4989da36. Should be thoroughly tested though, there still seems to be weird behavior in rare cases. I suspect it has something to do with a potentially longer partner train still exercising the buggy coupling.