I've found that in chassidish shuls (at least in London and Manchester, UK) that they always do one of the Shir hamaalos and a Kaddish before starting maariv with והוא רחום.
Is there a source and reason for this minhag?
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I found this article at www.daat.ac.il .
He gives several reasons:
Based on a gemoro in Brochos 4b [paraphrased]: a person comes in to the synagogue from his work in the field and he learns what he can learn and afterwards he says Shema and Amidah. So Chazal arranged that we should say Shir Hamaalos before Maariv in order to pray after saying Tehillim. (Shiltei Hagiborim on the Rif Brochos 8b and others.)
או"ז בשם תשובת הגאונים והכלבו, ד"ם או"ח סי' נד says from Kabbalah that one should not interrupt between Kaddish and Borechu. From this we see that we need to say Kaddish before Borechu. (Kaddish cannot be said without some prefatory pesukim (Kol Bo Siman 6 requires at least 3)), so Chazal chose to say Psalm 134 because it contains the words “who stand in the house of Hashem at night”.
And because Psalm 134 contains the words, lift up your hands and bless (וברכו) Hashem so we say this Psalm before ברכו.