Complex sentence: Originally, plans had been to number them E7001-E7010, but once rebuilt they were numbered E6101-E6110 instead, later being renumbered 74001-74010 under the TOPS system.
Split into simpler sentences: Originally, plans had been to number them E7001-E7010, but once rebuilt they were numbered E6101-E6110 instead. Later they were renumbered 74001-74010 under the TOPS system.