Most of what has been said here is true. In the UK over the past few years there have been New Boilers, cylinders, wheels etc been produced to replace the worn out components. There are several operation locos in the UK over 100 years old and a few from the 1870s. In the late 1980 ex Liverpool and Manchester 0-4-2 "Lion", the engine in the film "The Titfield Thunderbolt", built in 1835 was operational.
Though not Narrow Gauge The Bluebell railway has many realy old locomotives on there roster including 1872 built Fenchurch, operating with a 1920's boiler. Photos of her can be found on this link.