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There are now three copies of this at my local game club; it's a great game that keeps growing on me. Two of us picked up the big bag of expansions and have started using some of the assets to get a feel for how to help the Germans by just the right amount.
One question came up regarding the railroad flak: the flak stays on the "mobile" side until you choose to move it during the fighter phase, when you move it one hex in any direction and then turn it to the "deployed" side. We wondered if the flak is then immobile for the rest of the game, or can it be moved again? It seems pretty powerful if it can move every turn, but is only of modest value if it can only move once.
Either way, the expansions are a very nice addition to a very engaging game.
It looks like you're playing this correctly - the flak cannot be moved again after it is flipped to its "deployed" side.
The "Big bag of expansions" is a great addition. The 3-5 player expansion makes for some really fun possibilities, and, as you mentioned, the other expansions help to balance the game, which is naturally tilted in favour of the British player.