seunggangjange dochakhajamaja gaechalgu jjogeseo annaebangsongi deullyeosseoyo.

-자마자 means as soon as or right after doing something.

So:

• 도착하다 = to arrive
• 도착하자마자 = as soon as (someone) arrived

It shows that the second action happened immediately after the first one, with little or no delay.

In this sentence, it means that the announcement was heard immediately after arriving at the platform.

With -자마자, you attach it directly to the verb stem, not to a past-tense form.

So:

• 도착하다 → verb stem: 도착하-
• 도착하자마자 = as soon as (someone) arrives/arrived

Even though the whole sentence is talking about a past event, Korean does not usually put past tense before -자마자. The past meaning is understood from the overall sentence, especially from the final verb 들렸어요.

The verb 도착하다 normally takes the particle 에 to mark the destination or arrival point.

So:

• 승강장에 도착하다 = to arrive at the platform
• 학교에 도착하다 = to arrive at school
• 집에 도착하다 = to arrive home

For English speakers, this is similar to arrive at/in something. In Korean, 에 is the standard particle used with 도착하다.