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I asume you mean a 158/0..GWR have 158/9s (3 Car i/c 158798) and everything else is a member of the 158/0 sub class... 158/7 is merely a play on numbers and not a sub class
TFW seem to have been abandoning pasengers at out of the way locations for varying reaons with no information, no staff ,or such as oad transport developments
The 170 would have not gone to North Wales but would have changed diagrams for one reason or another, When this occurs the unit invariably remains on it's original in the system and catch up with events later.