when you walk into a fence tile, its changes its appearance to a post - as if it was next to a block.

before:

after:

using MC 1.5.2
Forge 737
railcraft 7.3.0.0
optifine 1.5.2_HD_U_D5

bug disappears when railcraft mod is removed

I just submitted a copy of this to the forge github...

Forge bug

but it works fine with forge installed but not railcraft, isn't that an issue with ure mod? any idea which forge version doesn't do this?

Its fixed in the latest versions of Forge already. Its because ChunkCache.isAirBlock() doesn't call the Block.isAirBlock() function.

ok so any idea which version of forge will work for MC 1.5.2?
as far as i was aware the last version was 738?

It might only be fixed in 1.6