Yes, according to Wikipedia the train is expensive and doesn't get used often. It's usually used when a royal has a very busy schedule and wants an easy mode of transportation and a place where work can be done and meetings carried out.
I highly doubt it. I don't think Camilla really cares what Catherine wears- she's never been super into fashion. Besides, Catherine seems to be very happy and comfortable in her recent outfits.
I'm actually thinking that Catherine was deliberately dressing up when Elizabeth was queen.