How easy is it to get a consistent RPM from steam power?
You won't. It is an old Mamod oscillating cylinder steam engine modified by turning the engine through 90 degrees. It is very similar to this 1949 model I had as a kid. It is still back in England I think.
I had a couple of steam powered toy locomotives and I built a crane powered by one of those Mamod engines as part of the lay out. There is no steam valve and speed is determined by load and steam pressure. Heat source is a methylated spirit burner. I can tell you that engine would also spit hot oil, water and steam over the record.
This is one of my steam trains, a working Bassett Lowke steam model of a Southern region Mogul steam engine. I last fired it up in 2004 for my grandson Ethan on a visit to the UK. It is still in my mother's house.
The other one was a Bassett Lowke Enterprise which my father had had as a child. It was like this one.
It was handed down to my nephew John, who I trust still has it.