Learn Hardcore Russian: Когда поезд подошёл к платформе, мы быстро вошли в вагон. - When the train pulled up to the platform, we quickly got into the carriage.

Because Когда поезд подошёл к платформе is a subordinate time clause: When the train pulled up to the platform. In Russian, subordinate clauses are normally separated from the main clause by a comma.

So the structure is:

Когда поезд подошёл к платформе = subordinate clause
мы быстро вошли в вагон = main clause

Even if the order changes, the comma stays:

Мы быстро вошли в вагон, когда поезд подошёл к платформе.

Why is it подошёл, not подходил?

This is about aspect.

подошёл = perfective, meaning the action was completed: the train came up / pulled up to the platform
подходил = imperfective, focusing more on the process: the train was approaching

In this sentence, the train reaches the platform, and then we boarded. That sequence fits the perfective verb подошёл very well.

So Russian is emphasizing a completed event:

first, the train arrived at the platform
then, we got in

Why do we use подойти with a train? Doesn’t it literally mean to walk up?