I read GT6 changelog and found that Railcraft tanks were disabled due to broken code. I wanted to enable them for my singleplayer game but I didn’t find such option in configs. Is there a way to enable them?

Uhm, have you checked if they actually ARE disabled or not? The Default Config Pack is just a set of Configs that you can install yourself, its like a set of Defaults for Modpacks, the disabled Tanks thing is literally the Railcraft Config for them.