[Piotrsko]

We did solid fuel back in the 1800-1900's. Wood, coal, whatever. Was however low density energy wise. I recall when they converted some of the loco's thay had to modify the fire box to make it smaller.

And there are periodic people making wood fired cars either by gassification or steam. Rather inefficient, however.

We also use solids as boosters strapped to the sides of rockets. Once lit, they are hard to un-light

I suspect that a solid fueled vehicle would be bigger and heavier than an equivalent gasser.