This is the 18396th most frequent Russian word.
электричку
Suburban commuter train.
In this sentence, 'электричке' is used in the prepositional case to indicate the mode of transportation, a suburban train.
Мы сегодня решили поехать на электричке на дачу.
Today, we decided to take the suburban train to our summer house.
Here, 'электричку' is in the accusative case because it is the direct object of the verb 'опоздал' (missed), referring to the suburban train.
Вчера я проспал и опоздал на электричку.
Yesterday, I overslept and missed the suburban train.
In this case, 'электричка' is in the nominative case as the subject of the sentence, referring to the suburban train that is scheduled to arrive.