You are very correct in saying that SAR crews preferred there Garretts to face bunker first in mountainous tunnel areas, but to my knowledge it was not any official policy. The more recent classes of Garretts had connections for a tank car on ether end so it mad no difference. The Garretts were very rarely turned as wyes and turntables suitable were very few. I can only recall one such facility, at Bullawayo in Zimbabwe. As Garretts are tank engines with the advantages of tender locomotives (in essence) anything was possible.