"She had a penetrating sort of laugh. Rather like a train going into a tunnel"

The quote is referring to a female's laugh, which is described as being "permeating". This recommends that her laugh was loud and could be spoken with a distance. The contrast to a train going into a tunnel even more stresses the loudness of her laugh, as the sound of a train getting in a tunnel is usually quite loud and can be heard from far. The quote suggests that the woman's laugh was so loud that it could be spoken with a distance, which it was an unique noise. It could be analyzed as an indication of her self-confidence and her ability to make her existence understood. The quote likewise suggests that the lady was not scared to reveal her feelings and that she was comfy with being the focal point.