I have used a couple but the easy to find (Walmart, Dicks, etc) Birchwood Casey Super Blue works OK on sintered steel. It seems to match closely. Not so good on sheet steel like car frames or diecast body parts like Post War steam chests. It leaves a definite bluish cast on those. Some of the lesser known products found in gun shops work a bit better on sheet steel but still not a perfect match for the hot blue that Lionel must have used.
Pete