Stopped at a traffic light. Light changed, I opened the throttle, and it sounded as if a 105 mm howitzer had been lit up right beside me. "*****", I said, as the drivers around me hit their heads on the headliners. Car didn't move. Loud hiss. Closed the throttle. Hiss stopped. No change in boiler pressure. (525 psi and rising.) No sign of steam anywhere. Opened the throttle. Loud hiss. No steam.
"Well," I thought, "Gotta be the superheater."

Got a push from the guy behind me, and pulled into the parking lot of the gym to which I was heading. Got a tow home, dropped the burner, and sure enough, a 2 cm hole in the superheater. Had a new one made, and with luck I'll post pix of the new and old in the next couple of days. Boy, 550 psi blowing a hole in a 1/2" mild steel tube makes a real bang. The new one is #304 1/2" SS pipe. Everything else will go before that does.

The blow-out is at the lower right of the old superheater.