On 27 Oct, 19:57, Jules
wrote:

What did Trevithick's efforts use, then?

It depends on which engine you're talking about and when: Charcoal
(the models), coke or coal. When the high-pressure engine was fixed
in place as a stationary engine it switched to burning coal, but it
also gained a far taller chimney that was sufficient to create the
draught it needed.

Trevithick was also using a centre flue boiler with a fairly large
grate area, which will burn coal tolerably well (although
inefficiently). The smaller-grate locomotives were those that had
particular trouble using coal, without forced draught.