Void.set("topBlock", "minecraft:coal_block")
Void.set("fillerBlock", "minecraft:coal_block")
Void.set("actualFillerBlock", "Railcraft:cube")

change to:

X= meta id
Void.set("topBlock", "minecraft:coal_block", x)
Void.set("fillerBlock", "minecraft:coal_block", x)
Void.set("actualFillerBlock", "Railcraft:cube", x)

should be very easy to do since u have this code with replace blocks.
also
"Void.registerGenBlockRep("minecraft:dirt", "railcraft:cube", 6)"
meta datas dont work on this code anymore, do not know why.

Not doing that for the set command modification. They already exist in the form of fillerBlockMeta and topBlockMeta for the set command.

I have no idea why you can't specify metadatas. They work for me.

becuase it doesnt work
can u full example of u using it