For those not familiar with AK Interactive, their focus is mostly with military model kits, paint and specialize in weathering. Their products are easy to use, even for the novice.
They have just released a book on weathering railroad equipment and building structures. The book has three main sections. Section one show you materials and techniques for weathering your models. Second, they walk you through weathering and finishing railroad equipment, locomotives and rolling stock from modelers around the world. Last, they discuss structures.
Be sure to visit their web site for full details.