Trevor,
"If I'm not mistaken under the right circumstances the combination of water and coal dust can form sulfuric acid which could eat right through the smokebox front."
This is very correct. Coal contains sulfer, which doesn't burn and is carried to the smokebox attached to soot, where it's mixed with exhaust steam. The acid won't be a concentrate like can be seen in labs that'll bore a hole through a person, but given time it will eat through just about anything includng iron and it's kin.
Curtis F.