Is there a "universal" flux core welding wire for repairing any kind of iron?
A railing ( tubular or angle) it it very likely to be "mild" steel , aka -carbon steel. It contains 0.1 to 0.2 % carbon , a little manganese and silicon. Highly weldable with standard filler metal . MIG is the easiest , gas shielded solid steel wire or flux cored steel wire ( the flux generates gas to shield the weld.). You want the most common standard  steel filler wire. You do not have the skill necessary to weld any type of cast iron; the railing is very unlikely to be cast iron." Wrought iron" is an archaic name for very low carbon steel ; unlikely to be in your railing but it would weld the same as mild steel.