When does x When do: Qual a diferença

Let´s put the sentences in the affirmative:
The train arrives soon.
The trains arrive soon.

It would work to any other noun:
Assuming that the cars arrive at random, find the probability that (a) no cars arrive during a particular five-minute interval,...
When the car arrives, you should have a chance to look it over again.

Or other verb:
Once the car achieves a good level of speed, the gas powered engine in the hybrid car will automatically engage...

Once the cars achieve a good level of speed, the gas powered engine in the hybrid car will automatically engage...

See? With plural nouns or not in the third person singular, the verb "loses" the "s".

Now let´s get to the drawing board, to the crux of the matter.

Since we are not in the affirmative, we use DO (to the plural or when not using the third person singular) with "trains/cars" plural forms.
With the singular form (train/car) it´s equivalent to the third person (he, she, it), so instead of the verb getting an extra "s" it changes to DO
DOES.

Hence with "train/car" etc, we use DOES, whereas with "trains/cars" we use DO. Notice that both - DO or DOES - "kills" the "s" of the verb (so with questions or negative sentences we will use DO or DOES - with affirmative ones it changes by the use of "s" in the third person singular.)

As a rule of thumb, you can think of

the car achieves
The cars achieve

So, the S "travels" from the plural noun (cars) to the verb (achieves if the verb goes singular), conversely if the noun loses the "s" (singular noun) then it "goes" to the verb.
It´s not what grammarians would say, I just am giving you a tip. It´s like algebra, the "s" wouldn´t be on both sides of the equation!

I hope it somehow clarifies things a bit.