Sources and Further Reading

Wiki Travel Guide of Rome:
most useful and comprehensive information includes plans of metro, trains, and buses

Catacombs Wikipedia
Brief illustrated descriptions of all catacombs open to the public

Churches of Rome Wikipedia
Chronological list of some 200 churches with Wikipedia descriptions

Recommended further reading:
Robert Hughes "Rome", A Cultural, Visiual, and Personal History, Alfred Knopf, 2011