I couldn't build smeltery (a multi-block structure from Tinker Construct) when I added RailCraft.
I built it correctly but it didn't work. When I removed Railcraft, it worked.
And one more thing, when I added Railcraft, there's an "ID missmatches" error.
I added Railcraft to the modpack, built a new smeltery (tinker construct) and it didn't work. The one which i built before i added railcraft works fine