I asked this question earlier. With your permission I just copypasted the answers I got.
>I can't make 9x9 Railcraft tank http://ftbwiki.org/Railcraft_Tank because of your phisics. How can I fix it?
>You can make the tank blocks stability have no physics or a stronger stability in the config files under rail craft. When you looking at the blocks. I believe you hit "Shift + J" to add the block to custom config... than either with a text editor or from the enviromine menu you can change the stability to something else. You can look at how glowstone works. You can also make your own stability types or read what types of stability their are available by default. The stability types are located in the stability.cfg. I am doing this by memory since I cant look at the moment.
> When I hit Shift + J nothing happens.
Shall config file be like this?
I:"02.Max Support Distance"=3
I:"03.Min Missing Blocks To Fall"=0
I:"04.Max Missing Blocks To Fall"=0
B:"06.Holds Others Up"=true
>that's the stability if you wanted to make a custom stability type You dont have to use it, if you just copy one of the others. what your wanting to do is go into:
"config\enviromine\profiles\default\CustomProperties" find railcraft.cfg (or something like that), and find or add the block you want ,and change its stability to the one you want... default ones are "loose","average","strong","none","glowstone",sand-like"
the ingame config is a bit easier I just checked it.. its J key + you hold shift.. look at the block with nothing in your hands. check your keybinds to make sure nothing has changed it.
it will look more like (Not tested may not work) In game config does most of this for you.. and than you just change the config file to point towards its meta id, and new stability type
S:01.Name=Railcraft:machine.beta I:02.MetaID=14 S:03.DropName= I:04.DropMetaID=14 I:05.DropNumber=-1 B:"06.Enable Temperature"=false D:07.Temperature=0.0 D:"08.Air Quality"=1.0 D:09.Sanity=0.1 S:10.Stability=none B:11.Slides=false B:"12.Slides When Wet"=false }
I hope it helps. Back then I finally built a tank.