I’m thinking more like having them exist in the world already. An have the engineers be warlocks pacted with the Iron Beasts “whose tracks impossibly reach both forward and back” as they travel the countryside.

“Their mighty roar is accompanied by quaking earth as they stampede into the outskirts of town. While the Iron Beast Tamers pull their loads from the beasts back, the Iron Driver feeds his beast coal to keep the eternal fire it it’s belly hot and smoking.”