What is the effect of ‘slowly, surprised, stopped.’ at the end of the fourth line of the first stanza?

Not Adlestrop, no – besides, the name
hardly matters. Nor did I languish in June heat.
Simply, I stood, too early, on the empty platform,
and the wrong train came in slowly, surprised, stopped.
Directly facing me, from a window,
a very, very pretty girl leaned out.

What happens to the pace of the line as you read it? What effect does the punctuation have?
Does the alliteration make any difference to the effect?
Who or what is ‘surprised’?