You may find that working in Trainz that prototype speed and signalling doesn't always work very well. It seems Alistair or whoever is driving your Trainz train hasn't been to that Netwrok Rail course....
For instance my latest rebuild of British Midlands2 won't allow a train to run towards a trailing point if the destination requires that train to reverse over it and enter a yard. the speed limit is 90 but nothing goes faster than 20. What I found necessary was to use a red marker as a 'target', run to that (which worked at full speed) then add the second destination. There are often quirky things like this with the AI.
This may be something to do with the built-in track direction, but that's only my guess, but I have found that you may well get a stopped train somewhere and clicking on the signal in front of it will give the message 'Track ahead is unsignalled' when you think it is or this odd speed restriction that shouldn't be there. So speed and signalling is probably linked.
Once we travel onto complex trackwork we have to use our own judgement as to where a speed sign/signal will need to go. If you don't want them seen use invisible ones.
Have you got right through to where you need to go? In other words have you laid all the track now? To get things moving and test use a 90mph, you can always replace it later and that way you can test the route to see how it performs and if you have hit any of the problems I sometimes encounter.
Angela