S-Bahn (so called because of it's resemblance to suburban metro railways in German, Austria, etc.), is often the major component of a PAX. Their purpose is to move the maximal amount of passengers through the city to a transfer hub, preferably using the minimal of space so as to maximise catchment. They are not always purely trains, road vehicles can also be used.

There is a blog article which covers this topic to the detail. Advanced Building Revue 05: Sbahns and city networks

Stereotypical characteristics of an S-Bahn are:

• Transfer, not unload, orders
• Highly compact stations and layout
• Short trains