たら conditional: verb+たら versus verb+ていたら

Question

In this video, a native speaker is walking around Asakusa and describing his trip in simple sentences for learners. As he's walking, he finds an old drinking water spout, and says:

「で、歩いていたら、昔の水道がありました。」

(I thought maybe he might have said 歩いて行ったら, but his own subtitles prove otherwise.) I'm unclear why he chose to say 歩いていたら, and what meaning this gives versus just 歩いたら.

I'm familiar with using verb+ていたら when the second clause is something that DIDN'T happen, like the examples below. But that doesn't apply to this sentence.

もし雨が降っていなかったら出掛けていた。

もし雨が降っていたら出掛けていなかった。

Even by thinking of 歩いていたら with "ongoing state" aspect, I'm unclear how that would translate naturally: "If you are walking, there was an old water spout." (?)

Can anyone explain?

Answer 1

It is the same たら as in

• テレビをつけたら、昔見た映画をやっていた。
• Other uses of the たら conditional

It expresses temporal vicinity rather than condition-consequence (though there is still a feel for it). From here:

(「～たら」の様々な用法）３、動作が完了した後（動作の連続）
＊前件の動詞が動作の継続状態を表していると、後件の出来事が、その動作の最中に起こったことを表します。

●家でテレビを見ていたら、宅配便が届いた。When I was watching TV, a parcel arrived.

So 歩いていたら means when I was walking.

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BONUS: There is a similar usage of と.

• S1 と S2 conditional S2 being in past tense

The reference in the answer says that sentences before and after と can have the same subject whereas it is not possible with たら. But this is not completely true: 歩いていたら昔の水道を見つけました is fine. So the difference may be even more subtle.