Periodic Overhaul.
The rolling stock and Locos on IR have a long life (up to 25 years in many cases). However many of the components used in them, like say the almost ubiquitous DC motors don't have that long life and reliability. also, things get old/outdated and up gradation is necessary at times, along with giving things a touch up to make them look new and presentable. all of these gets done during periodic Overhaul.
Think of it as a major renovation in your house, where you change your furniture, replace fittings in...
more... your home, go for a newer geyser and finally repaint your home. Your home may appear new, but it is still the same old home, only renovated extensively :)