On 28 Oct, 12:56, The Natural Philosopher
wrote:

Coal doesn't need a forced draught. Coke does.

Efficient burning of either needs a forced draught, which you can gain
from a moderate chimney.

Clean burning of coal, without a pall of black smoke, requires more
chimney height than is practical on a moving vehicle. Even back to
Queen Elizabeth's reign, there were laws against burning coal without
suitable smoke-preventative measures. As coke was widely known by now
for metallurgy, and any steam locomotive in this period was at the
leading edge of engineering, coke was an obvious solution.

The solution to burning coal on locomotives was to use the blastpipe
to increase the draught. This post-dates Trevithick, and was probably
invented by Hackworth. Coal may have been used by Hackworth at Wylam,
or at the Middleton railway (these locomotives were probably capable
of doing so, with excessive smoke) but the real shift from coke to
coal as a practical locomotive fuel is well documented through
experiments by Stephensons.