Learn Finnish: Odotamme lähtöä asemalla. - We wait for the departure at the station.

The verb odottaa (to wait for) always requires its object to be in the partitive case. Here, the nominative noun lähtö (departure) takes the partitive ending to become lähtöä.

Is the word lähtöä related to the postposition lähtien from this lesson?

Yes, they share the same root! Both are derived from the verb lähteä (to leave). Lähtö is a noun meaning a departure, whereas lähtien is an old verb form that has frozen into a postposition meaning since or starting from.

Why do we use the adessive case asemalla instead of the inessive asemassa?

In Finnish, public transit points like stations and stops traditionally take the adessive case. You wait asemalla (at the station) or pysäkillä (at the stop), because they are conceptually treated as open areas or surfaces rather than enclosed containers.