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While the pronunciation of str- as /ʃtʃr/ or /ʃtr/ (shtr-) is fairly common, it is less common than the simpler cases that generally go unremarked.

The reason behind this pronunciation is a phonetic process called Assimilation of Place of Articulation. In this case, the –r- sound is pronounced farther back in the mouth than the preceding sounds. The effect of the place of articulation of the r sound is that, in some contexts, its place of articulation can be assimilated by the preceding sounds. In the case of more forward consonants, they can be moved further back in the mouth. When that happens to the s sound, it can become /ʃ/ (sh).

The simpler case, which goes nearly completely unnoticed, is the case of tr- (and dr-). The sounds for tr- and dr- are even more widely palatalized than str-. So almost all of us pronounce train, treat, trim, trolley, truck as /tʃ/ (ch), as if they were spelled chrain, chreat, chrim, chrolley, chruck. The same is true of dr-: we pronounce drank, dream, drill, droll, drunk with palatalized d- or /dʒ/.

If you were to pronounce the d of drunk exactly as you do the d of dunk, you would sound foreign. Likewise with the t of truck vs. tuck.

But almost nobody remarks these allophones of t and d, which are just as remarkable and (not so) shtrange as str-.