Learn Russian: Трамвай, который сейчас едет в центр, работает без электричества. - The tram that is going to the center now works without electricity.

Который acts like an adjective, so it matches the gender and number of the noun it refers to. Since it refers back to трамвай (a masculine singular noun), it takes the masculine singular ending -ый. It is in the Nominative case because it acts as the subject of the clause (it is the thing doing the going).

Why do we use едет for the tram instead of идёт or ездит?

In Russian, vehicles use transport verbs rather than walking verbs. We use едет (going in one direction at this moment) instead of ездит (going back and forth, or habitually) because the sentence specifies the tram is going to the center сейчас (right now).

Why is the destination в центр instead of в центре?

Because the tram is moving towards a destination. When expressing motion to a place, Russian uses в or на with the Accusative case (the masculine noun центр doesn't change form here). В центре is the Prepositional case, which is used for static location, not direction of motion.

What case is электричества, and why did the ending change?