PR#540 - FIX: One train staff overlay is broken in diagonal direction

[RESTRICTED ACCOUNT]

The essence of the bug report in this thread was that the one train staff-only process did not correctly handle diagonal placement. Thus the code had an error checking for a tile without a track.
Thanks to james for addressing the crash in the interim.
However, the possibility of crashes had not yet been completely ruled out. The display was also broken, so I rewrote this part of the code and changed it so that it would display correctly.
Please check pull request #540.

[jamespetts]

Excellent, thank you for this. Now incorporated.