Biggest change from Steam to diesel was with Steam, you made all your own parts. Diesel, you call the local sales rep and they send you what you need. That cuts out a large chunk of the machine shop staff, as well as other departments (boiler workers, pipe fitters, sheet metal workers, ect) because most of the parts become off the shelf items. Part of this was due to diesels being built the same for various customers, where as steam locomotives were built to order for the customer. So stocking items became possible.