The 4mm Peco card wagon kits are being sold again but not the metal inside bases for them, for some reason. So is the wonderful wagon underframe, usable for 9 ft wood or 10ft wb steel u/f. Interesting technology there if they are using the sme flexible plastic.
See http://www.peco-uk.com/prodtype.asp?strParents=3309,3322&CAT_ID=3325&numRecordPosition=1
and I found examples on sale at Kernow models.