Hi AntonyVW
Unfortunately your post is rather vague.  Are you talking about semaphore or colour light signals, scripted or unscripted?  What is the situation that is causing you problems?  Are you modelling UK or other countries railways?
I normally model UK practice and find that the scripted signals, both colour light and semaphore, are generally very good and work realistically.  Having said that I use the Path Rule and driver commands which give precise control over where the train is going and avoids tha AI having to plot its own route.  If you are having problems then it could be a number of things such as not setting the route soon enough, not setting the route far enough ahead or incorrectly placed signals.
More information would help.
Regards
Brian