The teacher who sets the transcription task has to exercise a certain discretion when correcting the student’s work. Students are encouraged, after all, to represent their own accent rather than some variety different from their own (though in Britain we normally allow them to choose to transcribe RP if they prefer, even if it is not exactly their own accent). Where do we draw the line between a mistake on the one hand and a permissible local-accent feature on the other?
Consider someone who transcribes train as tʃreɪn, drink as dʒrɪŋk etc (which is by no means unusual).
What I would do faced with this is to draw the student’s attention to the fact that the usual transcription is treɪn, drɪŋk. I would mention the allophonic rule that makes English r fricative in the clusters tr and dr, and also the corresponding allophonic rule that retracts t and d when followed by r. I would then try to get the class to discuss which analysis is right, and why.
Do the initial clusters of train and drink contain any phonetic matter that cannot be attributed to the fricative r that we expect in this context? Is the friction element followed by the r element, or simultaneous with it?
The concept of phonological neutralization is relevant here, though sometimes difficult for practically-oriented students to grasp. The point is that there is no possible contrast in English words between a posited cluster tr and a posited cluster tʃr. This means that the opposition between t and tʃ is neutralized in the context _r, at least word-initially, and likewise the d - dʒ opposition. So in a sense it is meaningless to ask which is involved.
It may be relevant to consider the pair century – sentry. Obviously, century is basically ˈsentʃəri and distinct from sentry ˈsentri. However, like other words with this phonetic structure, it is subject to optional compression in the form of the loss of the schwa, leaving ˈsentʃri. Is this still distinct from sentry?
If the answer is no, they are not distinct, it confirms our diagnosis of phonological neutralization. If it is yes, they are distinct (which it tends to be), then we ask whether ther initial affricate of train is like the -tʃr- of compressed century or like the -tr- of sentry. It is like the latter, and we transcribe accordingly.
You will now see why I was not convinced by Joshua Smiles, who wrote to me asking
I wonder whether you might know the name of a phonological change occurring in London speakers of RP (and those who simply watch too much television). What I refer to is the shift of any alveolar plosive preceding a rhotic consonant to a post alveolar affricate. Examples of this would be "tripoli", and "children" which are often rendered /tʃɹɪpəli/, and /tʃɪldʒɹən/ in RP and EE alike.
I don’t believe there is any such phonological change in progress. So naturally I don’t have any name for it.