Timer Events and the Resource Manager
nTimer in the Resource Manger is fired every half second
nTwo arrays keep track the current status of the photocells and the previous status of the photocells
nWhen these two arrays differ we need to handle an arrival or a departure of a train. 
nOn an arrival the system finds and if possible allocates new resources to a train.
nOn a departure the system frees resources no longer needed by the train.