Jules wrote:

What did Trevithick's efforts use, then? I did did some googling but
couldn't find any mention of the actual heat source used.


The first run was on 21 February, and was described in some detail by
Trevithick:

"...yesterday we proceeded on our journey with the engine, and we
carried ten tons of iron in five wagons, and seventy men riding on them
the whole of the journey... the engine, while working, went nearly five
miles an hour; there was no water put into the boiler from the time we
started until our journey's end... the coal consumed was two hundredweight".

http://www.museumwales.ac.uk/en/rhagor/article/trevithic_loco/

--
Rod