Hi again Stefano.
I want to be sure which heralds you are referring to.
First this is only on zebra stripe locos, correct? If so then Jeff's info, while correct, might confuse you as he is referencing multiple paint schemes and not just zebra stripes.
I figured some photos would help out. Both are modern paint jobs on museum locos, but both are helpful, and in color...which sometimes is hard to obtain for zebra stripe locos.
"black cross in white circle"
"blue cross in white circle"
Both logos are correct, and to be honest I'm not sure if anyone has ever figured out which should be applied to a particular loco without a photo to back it up. And then you really need a color photo to be sure. A lot depends on where they were painted. Some locations used the blue, while others used the black.
Interesting info is that blue version was supposed to be holographic, appearing blue at one angle and black at another. My Life-Like Proto 2000 GP7 (zebra striped) has this holographic effect. Pretty cool.
Oh and those zebra hand rails on the first photo (loco 2301) are not prototypical. At least I don't think they are. Haven't seen any photos of this on Santa Fe locos in photos of the correct era.
Hope this helps.
To confuse you even more with zebra stripes, some locos had the ATSF lettering with periods after the letters (A. T. S. F.), some included an ampersand (A. T. &. S. F.), while others just had ATSF, no periods, no ampersand.
And even more confusion, some zebra stripe locos had white stripes, while others had silver/aluminum stripes.