I grant that "train" may not be the best translation for the crucial verb in Prov. 22:6. May I suggest (dynamic equivalence) "baptize"?
The verb is hanak. It is the verb from which we get the derivative Hanukkah, which means dedication or consecration. It refers to inauguration or dedication, usually in a cultic setting. The dedication of Solomon's altar took seven days (2 Chron. 7:9). The temple of Ezra's day was consecrated in a similar way (Ezra 6:17). It can even be used of the dedication of private houses (Dt. 20:5). The rededication of the Temple in the Maccabean period gave that holiday the name used for it today.
So, admittedly, train is not the best translation, but a better translation hardly weakens the point I am seeking to make.