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I wouldn't rely on someone remembering they need to open it for you.
Last time this happened to me I just blocked the door from closing again and again until someone came down to try and work out what the problem was.
They were really annoyed at me until I said I had a bike locked away and had a proper bike booking for the journey, so it was their fault for not being around to open up for me at my booked stop.
Yes, they use the old Virgin trains. I wouldn't rely on someone remembering they need to open it for you.