IIRC, the terrain designer chooses which type of re-supply each field gets. So as the maps rotate, check for them. <S>
Hunting trains with a P-38J in 20th FG markings was some of the best non-dogfighting fun I ever had in AH. Especially if you went low and clean, just strafed 'em. Quite fun and challenging, especially when they could shoot at you from inside a hill.