Not much higher. The Red Devil is claimed to have reached 15.9% once, which remains the record for a normal locomotive. The machines built in the nineties by Swiss company DLM do 13%. As for future projects, check out the Advanced Technology Class 5AT 4-6-0 -- they plan for a 14.1% thermal efficiency.

Note though that various special designs (fireless, high-pressure and condenser steam locomotives) can go higher.

_{*Lunatic*, n.
One whose delusions are out of fashion.}