What is a cast-iron schedule? [closed]

Significantly, cast iron is one of the least-ductile of metals. That is, it is very stiff, and if you try to bend it, it breaks. Cast iron must be cast (formed) into its final shape while it is molten metal, whereas metals like aluminum, copper, or even normal iron-based steel can be bent or "formed" to some extent while at room temperature.

So, anything that is "cast-iron" cannot be bent, and saying a schedule is "cast-iron" is saying "You ain't gonna change it, baby." A more familiar idiom with the same meaning is "cast in concrete".

I'll note that "schedule" is used in manufacturing to refer to specific specifications. For instance: https://www.metalsupermarkets.com/what-do-pipe-schedules-mean/

Whether this meaning is alluded to in the original quotation is hard to say, though.