A dead line is x number of yard tracks where locomotives can be stored..The reasons could be for the following:
1.Stored until needed*.
2.To be traded in on new locomotives.
3.Sold to locomotive re-builder.
4.Sold to another railroad.
5.Stored until they are rebuilt.
6.Sold to a scrapper.
*Some railroads (especially class IIs) sees down turns in business and store surplus locomotives till business picksup.Some slow down in business could be cause by seasonal traffic such as grain,oats,corn,ore,lumber,stone etc.