Simplify the following text for a Grade 6 native English-speaking person.
Original: Originally classed as AM4 units , they later became Class 304 under the TOPS numbering system , and could be found in operation over most of the southern West Coast Main Line .
Simplified: They were at first classed as AM4 units and later became Class 304 under the TOPS numbering system . They could be found in operation over most of the southern West Coast Main Line .