に乗ってた

was riding on

→ に
phrase Rare #25,426
was riding on A casual contraction of に乗っていた, describing being on or riding a vehicle in the past.
JA: 昨日はバスに乗ってた。
EN: I was riding the bus yesterday.

あのバスに乗ってたよ。

I was on that bus.

トムはメアリーとバスに乗ってたんだ。

Tom was riding the bus with Mary.

列車に乗ってた?

Were you on the train?

電車に乗ってた?

Were you on the train?

同じバスに乗ってたの家までずっとついてきたのよ

The person who was on the same bus followed me all the way home.

— バジュランギおじさんと、小さな迷子 (2015)

旅行中はどんな自転車に乗ってたの?

What kind of bike were you riding while you were traveling?

エレベーターに乗ってたあなた

You were in the elevator.

— エブリシング・エブリウェア・オール・アット・ワンス (2022)