Rails have much more limited capacity. Train costs in the UK are mostly dominated by demand exceeding supply, and supply being hard to add. Also you don't have

Rails have much more limited capacity. Train costs in the UK are mostly dominated by demand exceeding supply, and supply being hard to add. Also you don't have to maintain tracks in the air, and airlines have less militant unions. The final big hammer is of course competition. Airlines have lots, rail companies don't.