[davidpapermill]

Are the trains located where the urban areas are, or are the urban areas built around the train network?

It's chicken and egg question, but in Manchester and London it's very clear that mass transit led to urban development, rather than the other way around.

It's very surprising that cities like Leeds have no mass transit at all, and sizeable cities like Liverpool and Birmingham don't have much.

[ascorbic]

Unsurprisingly they mostly run trains to places where people live, but there is some influence the other way too. New commuter rail lines have historically lead to more development around the stations (e.g. Metro-land 100 years ago https://en.wikipedia.org/wiki/Metro-land), and in cases where a station is built for reasons other than there being a centre of population (e.g. because it's an important junction) then that does also tend to lead to houses being built. Of course in more recent times this hasn't happened because you can hardly ever build outside of existing conurbations, but that is changing and several new towns are being planned for new or existing train stations.

[rsynnott]

A bit of both. Adamstown, a new (2006) suburb of Dublin, is a very obvious example; it was planned on the basis that it was on a train line, and a station was built as part of the initial development. Less dramatically, you tend to see densification around train and tram networks.