At least two small tourist steam loco's were running on
LPG in Queensland some years back. LNG would be the
way to go if the Liquefying Plant was near by.
LNG has one good safety point over LPG. LNG is lighter
than air and therefore rises, unlike LPG which is
heavier than air and can leak into all sorts of odd low
lying spaces just waiting for an ignition source.
However LNG has several disadvantages, it is not
readily available and requires special handling and
transport.
Why not convert to oil burning.? Steam Ranger in South
Australia fired their ex SAR oil burners on waste oil
from the State Government fleet, basically cost them
nothing, you can not get much cheaper than that.
Of course the greenies might object.!
Cheers
----Tell