Back to Oisin88's suggestion - a very quick easy cheap interim fix is movable barriers - ropes or the metal ones.
Platforms 6/7/8 are the worst, because people ignore the queue and sit on the seats near the top then just barge in when the doors open. Barriers would sort this out and I have seen them used once or twice.
Platform umbers coluld be stuck on the barriers so people know what they're queuing for as people often have to walk to the top of the queue to see what it's for or ask others.
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