Earthwork refers to Excavation and Laying of Bed which includes cutting and filling to give a proper profile and for giving support to the Rail Track.
Strengthening of Major/Minor may refer to multiple types of work depending upon the problem in the existing Structure. Some Girders/Slabs have to repaired/replaced in case of Crack. Elastomeric Bearings may need to replaced which help Girders to Expand/Contract due to temperature changes.
Dismantling of track in general term which refers to removal/cutting of track.
Ballast...
more... Stone Work is the Stone laid around the track which absorbs Vibrations from a Fast Running Train. Delhi Metro has no Ballast and Vibrations are controlled by strong RCC structures in that case.