[susan53]

The subject of the sentences are "the bus" and "I".

However, notice that these sentences are not the same - again, you've got two separate questions here. So am I is one sentence type, whereas here comes the bus is another - it's the same as the sentence from your other question : Here we go.

So am I

There's nothing strange about the verb form in So am I -the verb used reflects the verb in the initial idea - whichever verb would be the operator of the sentence is used. (An operator is a verb that can perform a grammatical function such as carrying the negative or, as here inversion. So basically the verb BE plus all the auxiliaries)

A : I was in Rome yesterday.
B : Really? So was I.

A : I must go now.
B : Yes, so must I.

A : I've already seen the film which is on tonight.
B : Yes, so have I.

A : I'm really hungry.
B : Yes, so am I.


Here comes the bus

In the second sentence type, however, the verb is strange. Instead of the present simple in Here comes the bus! you would expect the present continuous, as the meaning is Look! The bus is coming!

But if you look at this sentence you'll see the re are several strange things happening : The "fronting" of the adverbial; the inversion of subject and verb; and the verb form. This all happens when there is an adverbial of direction (here, there, up, down, over etc) and a veb of position or movement (come, go, fly, stand, lie etc)

Here are some more examples :

There goes John!
He pulled the switch and out went the light.
Up flew the kite!
On the cliffs stands an old castle.

1. The fronting of the adverbial of location - this often happens for emphasis or to maintain the "given-new" information structure of English. Eg in

Greatham is famous for its high cliffs. On the cliffs stands an old castle which was built in 1498.

Here the given-new structure is maintained. The information "the cliffs" is "given" by the first sentence and therefore appears first in the second sentence followed by the new information "a castle". Turn the information order around and it sounds strange:

Greatham is famous for its high cliffs. An old castle stands on the cliffs ..

2. When the adverb of location is fronted, if the subject is a noun phrase, and the verb describes position/movement the S + V can be inverted. But this doesn't happen if the subject is a pronoun. So :
There goes John! / Up flew the kite.
but
There he goes. / Up it flew.

3. When the adverb of location is fronted like this, only simple verbs are used, whether there's inversion or not.