《アパート『へ』帰る『と』ハガキ『が』一枚届いていた…》

In that sentence, why is the verb using the past progressive form? 届いていた=〜て+いた
right? Does it mean that the letter was being delivered when he got to this
apartment?

Source:
[ブラックジャックによろしく](https://www.sukima.me/bv/t/blackjackniyoroshiku/v/1/s/3/p/10)
CC BY-SA 4.0
2020-05-02T12:11:44.037
76995
2020-05-02T12:11:44.037
37089
question
grammar translation syntax て-form tense
Japanese Past Progressive?