It probably falls into "questionable" category. Sorry, only Subway really knows what it puts in its bread.
From Islamic and Nutrition Council of America website:
All foods are considered halal except the following, which are haram:
Swine/pork and its by-products
Animals improperly slaughtered or dead before slaughtering
Animals killed in the name of anyone other than ALLAH (God)
Alcohol and intoxicants
Carnivorous animals, birds of prey and land animals without external ears
Blood and blood by-products
Foods contaminated with any of the above products
Going down the list, I doubt if bread will have any meat or alcohol, so it rules out pretty much everything except "blood and blood by-products." It is possible, although somewhat unlikely, that the bread will have some enzymes or emulsifiers derived from blood products. Again, from IFANCA site: "Foods containing ingredients such as gelatin, enzymes, emulsifiers, etc. are questionable (mashbooh) because the origin of these ingredients is not known.
"
I personally think the odds are pretty small. Since it is ok to eat something that reasonably appears halal, even if it ultimately turns out to be haram, I say it's probably safe to eat it.