Just about all the signal systems in the US are being revised as PTC approaches. Is the signal in question facing the train or away from the train (i.e. facing what would be an apposing train?) If the signal is facing the train as it is approaching, then it should be lit. If the signal is facing away from the train and lights up once the train passes into the block the signal is facing then it is o.k. They have been changing to approach lit signals, and if the block is empty that the signal is facing then in won't light up until something occupies that block. This is to save energy and (mostly) to extend bulb life. It is simply a more modern system.