Very interesting and precisely what I have found on the gradual development of the three mono tube steam generators I have ruptured on my Doble steam cars, now replaced with a fourth.
The only way I could tell that the tube had ruptured was a small amount of water under the steam generator, being unable to raise any more steam even though the fire was on and a very faint hissing sound. All three steam generators were operating at 900 psi, the working pressure of Doble D2. The E types, for your information, operate at 750 psi.
The cure to my problems has been to change from a stainless steel steam generator that came with the car to a gradual increase in tube thickness arriving at the top three coils now being 1/4 inch thick and 1 1/4 inch OD, the rest being also thick walled, all low carbon steel.
A number of Doble owners have gone through this process and all say that it is difficult to tell when the steam generator ruptures and confirm the above.