Realistic Imperfections
No city is completely clean or without defects. Therefore, it is useful to create scenery that has imperfections. It can only add to the realism of the layout. Trash and vandalism are commonplace elements in cities, so they can be interesting additions. Things like small, torn pieces of newspaper, garbage cans and graffiti will add to your cityscape realism. You don't need a completely littered city, but when you add some of these urban elements, the layout will seem more real.
Choose a Real Location
Another thing to consider is the location of your scenery. You might have a particular country in mind or a specific city. You also might consider a rural locale. A particular place can be a good idea because then you will have plenty of reference material for it, such as existing photographs. If you choose a specific city or town, you can consult photographs for details, and if you have visited the city, you can even add elements you've noticed yourself.
Consider the Season
Once you've established some realistic touches in your chosen locale, another thing you can do to bring your layout to life is to add an element representing one of the seasons. If it is fall, then you could add some bits of leaves beneath the trees. By choosing a specific season, you'll know what to use for full effect.
Be Creative With Materials
When working on your scenery, resourcefulness is an advantage. You can be creative with materials that will simulate desired items. Near railroad tracks you might want to add the element of gravel. To do this on such a small scale, real pebbles and stones will be too big, so you can use nuts for this effect. By mashing up the nuts and placing them where you want them, you'll achieve the desired effect. All of the nuts will be different too, just like real pebbles.