Does the boy's growing sense of isolation have, also, moments that are almost triumphant?This is from discussion part of my text book. I couldn't...

You are certainly right to identify this part of the story
as key in helping to establish the theme, however I think you are somewhat misguided in
your assessment. Let us have another look at this passage and let us also remember that
this paragraph comes as the boy is finally able to go to Araby and fulfill or accomplish
the noble quest he has been given by Mangan's sister. Let us also remember that the boy
frames this quest in obviously high-blown and Romantic
terms:

I imagined that I bore my chalice safely through a
throng of foes.

This is almost Arthurian in its imagery and
the boy obviously casts himself in the role of knight-errant fulfilling the quest of his
lady.

However, as his journey begins, the description of
what happens and what he sees irony mocks his notions of nobility and
Romanticism:

I
took my seat in a third-class carriage of a deserted train. After an intolerable delay
the train moved out of the station slowly. It crept onward among ruinous houses and over
teh twinkling river. At Westland Row Station a crowd of people pressed to the carriage
doors; but the porters moved them back, saying that it was a special train for the
bazaar. I remained alone in the bare carriage. In a few minutes the train drew up beside
an improvised wooden
platform.

Note the irony here
- the train is "deserted". It is apparently a special train for the bazaar, and yet the
boy is alone - no one else wants to go on this train. It is described as "bare" - hardly
appealing or attractive. When the train finally arrives at the bazaar, it draws up
beside an "improvised" platform - obviously a detail that emphasises the rough and ready
nature of the bazaar - hardly the centre of romance and mysticism the boy is hoping for.
Thus such details as you have observed do not add a triumphant note to the boy's growing
sense of isolation. Rather they pave the way for the epiphany at the end of the story,
when the boy realises that he is a "creature driven and derided by vanity" and he comes
to terms with the childish nature of his hopes and dreams.