Bill, it seems inbelievable, but is is true: Some of these lokies really lasted one whole working day, others had only one or two partial rechargings - not more.
They were charged (10 to 30 minutes) in the morning. Now the trick was to fill the reservoir (see, it's not a 'boiler'

) up to 85% with boiling water, which was then heated up to about 400°F (about 205°C) by superheated steam. The pressure went up to 400 psi. (But since there was no further heating, there was no danger of boiler explosions!)
Now the physics behind it: As soon as the throttle was opened and the pressure dropped a little bit, some of the superheated water flashed into steam. While the engine used up some steam, more of the water was transformed into steam, and that's why the relatively small reservoir could deliver power for a long time. The pressure slowly dropped, but their steam engine was designed to run in a wide pressure range down to 10% of the initial pressure! Of course they were heavily insulated, that's why the look somehow 'fat' - sort of a rolling thermos bottle.
Fireless locos look somewhat primitive - no big stack, no firebox, no injectors and all that stuff. But somehow they were 'high tech' products in their day - ideally suited for places where sparks were intolerable (e.g. textile mills, powder works) or noise wasn't wanted (switching moves in streets, all you heard was a soft exhaust hiss).
Ron
You find lots of informations
here (a NRHS page). Here's another fireless Porter from that page: