Well, passenger rail was a way (initially) for freight companies to advertise their freight services. The nicer the passenger accommodations/ quarters, the better they thought people would view their bread and butter services. Remember rail was once very exclusive and expensive. As the car became more important (and available) passenger service began to decline. Freight maintained it's status as important as trucks initially had a very small capacity and trains delivered goods to the hubs, allowing trucks to work the spokes. As companies started loosing money on passenger service, they abandoned them (pretty much where we are at today with the exception of Amtrak). When it came to cities there were certain constraints in the eventual introduction of rapid transit systems. Mainly street width (for cut and cover or above street lines). So rails were necessary and also the available technology of the day. In some cities such as London, tunnels were built deeper due to a mess of service lines and a desire to offer protection to residents, but these lines were often extensions to existing rail lines. As cities determined areas needed service, well the subways went where they had to. Finally, frequency has to do with demand and budget. Ornamentation has to deal with a decision on the scale of functional (and more cost effective) to functional and a showpiece and cities making a decision on where they were to fall. In short, rapid transit (rail type) systems exist because it's what we had and what we knew, they are expanded because it's what we have, and they are built from scratch because they are seen as metrics of a modern city (even if they are a less than modern amenity). Excuse typos!