Not real sure if you are looking for a kit to manufacture decals (the media) or the software to create art for decals. There are many variables in making decals and what end result you are looking for.
The first thing no matter what is a vector art program. Without good art work your result will be less than satisfactory. There is no need to spend a lot.....you can get older CorelDraw programs like V8 to V13 for under $40. There are some other but I do not use them so others may need to recommend. Good art is the start to good decals.
After you have good art you can print them. The biggest problem is your home printer will not print white....a mainstay for railroaders. And the gold a home printer will print, another often used RR color, is pretty poor. You can use decal paper that has white ink printed across the entire sheet but that causes issues too.
If you could give a littel detail on what you are looking for it may help. And you may also want to search the forum here as this has been covered many times. Good luck.