Railroad board is just a type of stiff cardboard that comes in different colors. If I recall correctly, I used black 4-ply board to make the ribs for my bellows.
BK-5 is fine, but you need multiple layers. I use it and wilderness outfitters 'silverized taffeta' material for my wet plate darkbox... Works slick.
Crisf Thanks for the link to the bellows material
Andrew- I tried to google wilderness outfitters and got camps and guides and thing like that-no apparent stores
Do you have a link you could please share?
IIRC "railroad board" is a card with a recycled core. I think a far better material (considering what it is going to be used for) would be something like Strathmore illustration board. It is cotton and very durable. I think the extra cost would be well worth it.