Complex sentence: Out of the 18 Non AC rakes, 11 will be phased out gradually since they have expired their normal life of 25 years, since the remaining 7 rakes are still in better condition they will be refurbished and will be pressed into service in phase manner.
Split into simpler sentences: Out of the 18 Non AC rakes, 11 will be phased out gradually since they have outlived their normal life of 25 years. The remaining 7 rakes are still in good condition so they will be refurbished and put into service in a phase manner.