Although TLG could probably sell a lot of sets with train accessories, and make money from them, there is a fierce competition for shelf space in stores these days. If TLG was to replace a mainstream fire truck set with a train carriage, it would not just have to sell well, it would have to sell REALLY well. Lego brand stores can provide a wider range of products, but there is a limit even there, not the least from a logistics point of view. However, it should be possible to cater to niche markets through S@H...