While there are many different types or grades of staninless steel, almost all railroad cars use either 301 or 304 stainless steel.
Per Wiki,
Type 304 is the most common grade; the classic 18/8 (18% chromium, 8% nickel) stainless steel. Outside of the US it is commonly known as "A2 stainless steel", in accordance with ISO 3506.
Type 301 (17% chromium, 7% nickel) is highly ductile, for formed products. Also hardens rapidly during mechanical working. Good weldability. Better wear resistance and fatigue strength than 304.
Initially Budd was using Type 304, made by Allegheny Steel. Type 301 was developed later and became popular with railcar manufacturers.
Both of these grades are corrosion resistant and will not rust easily.