Using Light Dependant Resistors (LDR)

Hi All,

In a couple months I am doing some modelling at an exhibition and I want to make a signalling circuit that connects to colour light signals so the aspect changes when the train passes. I know I could use a Heathcoat Electronics system but I want to be able to build circuits while having a demonstration one running. I've read about using Light Dependent Resistors but haven't seen many circuits relating to them and signalling.

I would envisage that on the day I have a train on a shuttle loop with the signal and connected circuit on one track and the train running past the signal (changes the aspect) then switches the points and it passes on another track giving time for the signal to change back and then the sequence begins again. Let me know if a diagram is necessary to explain all that.        
I would prefer not to use reed switches as that involves fitting magnets to trains. What circuits and components would be necessary to achieve this. This does not include the shuttle and point switching products as I have access to them already. 

Regards Connor