Ok Jonatan, that works for me however.
But perhaps you should check your graphic card options (via the driver itself or Nvidia Control Panel or ATI/AMD interface, or via Riva Tuner or similar) and see if your mip-mapping is not set on "forced on". You usually have 3 options there like "always forced on", "always forced off" and "defined by the application". For allowing the in-game mip-mapping setting to be effective, you usually have to set your driver/graphic card options on "defined by the application" (well, that's obvious! sorry
Also, in these graphic/driver options, you usually find a setting about mip-mapping quality as well. Several steps from "high performances" to "high quality". Select "high quality" in order to have the mip-mapping effect as reduced as possible and that should help about resolving that railing alpha textures issue.
At least I hope it will work in your case, since it logically works for me regarding my 4 large ships that use alpha textures for railings.
EDIT: I think -perhaps, not 100% sure- you must use at least the shaders version 2.0 in your graphic driver as well (depending on your graphic card if it supports it -but it should since shaders V2.0 are quite old now-) So if the shaders version option exists in your driver, then select it (or 3.0 if available).
If all the above doesn't work, then try to play with the other graphic driver' settings like "transparent texture anisotropy level" and all.