'-ㄴ' and '-던' have the same function in terms that they refer to the past event. However, naturally, '-던' goes with verbs related with duration of event, and it doesn't comebine with verbs associated with semelfactive aspect(or temporay event)
For the sentence as 'There are people who arrived an hour late.', if you express it in a sentence like (1B), it would be regarded as very unnatural.
(1) A. 한 시간 늦게 도착한 사람들이 있다.
B. ??한 시간 늦게 도착하던 사람들이 있다.

When '-던' sticks to durative verbs, it has the smilar meaning like '-ㄴ'. But, '-던' emphasizes something continued in the past more than '-ㄴ'. The following (2A)and (2B) refer to somethng in the past, but (2B) puts the emphasis on a duration.
(2) A. 이 집은 내가 어렸을 때 산 집이야.
This house is where I lived in my childhood.
B. 이 집은 내가 어렸을 때 살던 집이야.
This house is where I used to live in my child hood.

On the other hand, '-었던' tends to be used for repeitively continued action of certain period. In the following examples, (3B) or (4B) is considered more natural than (3A) or (4A) to refer to repeated duration of actions.
(3) A. 이 커피숍은 전에 자주 간 커피숍이다.
B.이 커피숍은 전에 자주 갔던 커피숍이다.
This coffee shop is where I would go a lot.
(4) A. 이 책은 내가 자주 읽은 책이다.
B. 이 책은 내가 자주 읽었던 책이다.
This book is the thing I would read a lot.

(The translation of the sentence is only for helping the understanding of Korean sentence. You shouldn't be confused.)

But, '-ㄴ' and '-었던' have no difference in their meaning when combined with verbs having property of temporariness rather than duration,

(5) A. 이 커피숍은 바로 전에 우리가 간 커피숍이잖아.
B. 이 커피숍은 바로 전에 갔던 그 커피숍이잖아.

(This coffee shop is where we visited earlier.)

(6) A. 이 남자는 바로 내가 어제 얘기한 그 남자다.
B. 이 남자는 바로 내가 어제 얘기했던 그 남자다.

(This man is the one I talked to you about yesterday.)