The EUREKA is an 1875 Baldwin narrow gauge 4-4-0 that was built i 1875 and is still going strong. The Nevada State Railroad Museum in Carson City, Nevada has the INYO a standard guage 4-4-0 also built in 1875. It is run every so often as well. Both still have their original boilers.
In Washington DC at the Smithsonian they have a locomotive there built in the 1830s, as I recall, that they take out on rare occasions for a romp down the rails.
How much of it is orignal I cant say.
The limiting factor, other than regular maintenance, is usually the materials of the boiler. Steel is good, iron is ,well, not so good.