Some thoughts regarding setting, feel free to use or ignore. This assumes a 20s or 30s setting.
Zenith is the second largest city in the Union, and largest in the state of Sinclair*. It's a booming industrial town on the shores of Lake Winnemac, gateway to Canada and the St Laurence Seaway. It was built on a foundation of industry, as a great crossroads of railway links between the East and the Great Midwest. Aside from railroads, other major industries are steel, meat packing, and shipping. It's booming breweries and distilleries are threatened by the onset of prohibition. It has a downtown business district of glamorous new skyscrapers housing its great banking an insurance houses, along with the corporate offices of its great industrial firms. Fortunes are being made in construction.
Zenith is a diverse city, and its rapid growth has drawn immigrants in droves. While the North Heights are home to gracious, refined manors, much of the city is given over to slums. These are broadly broken up by ethnicity. 'Paddytown' is an entrenched neighborhood dominated by blue-collar Irish, many second or third generation and striving for legitimacy and acceptance into the middle class. Little Palermo, across the Minnimac River, is home to the more recent Italian immigrants. The Cleveland Bridge links the two banks of the river together physically, but Cleveland Avenue is the dividing line between Paddytown and Porter Flats, home to most of Zenith's black population. On the Eastside south of Cleveland Avenue is home to the Jewish Ghetto, which is nearly surrounded by neighborhoods of immigrants from the Slavic nations of eastern Europe.
There's a small Chinatown near the docks, and respectable middle class suburbs of Maple Ridge, Eastchester, and Brighton are linked to the city by commuter trains.
Footnotes:
*Zenith is the city from Sinclair Lewis's novel Babbitt, and the state of Sinclair is named after both Sinclair Lewis and Upton Sinclair, author of The Jungle
Obviously I added some populations. A major American city, especially in the industrial Midwest, is scarcely complete with out a Slavic (particularly Polish or Russian) neighborhood, as well as a starkly defined black neighborhood and its accompanying simmering racial tension. A Chinatown adds the option of having a local Triad affiliate.