Improvements to logic for AI train when following other train, situation was not always processed correctly.
Also improvements to logic to test for train ahead, function sometimes returned incorrect values if both trains are in same section.
git-svn-id: http://svn.uktrainsim.com/svn/openrails/trunk@2751 a9f3cfff-6c41-7446-b0ca-052bd761e6eb