I've seen this sticker up at train stations. It reads, quite simply and to the point, "Walk your bicycle on the platform"
They could have added a 'please' or a 'would you mind terribly...' but the graphic design helps soften the tone. A bit of Cycle Chic feel to it, but she must be holding that male city bike for her boyfriend who had to pop into the bathroom before the train came.
In three and half years of documenting this bicycle culture lark I've only ever seen one person riding on a train platform so I really can't imagine that it's a problem. The one I saw is right here:
And that makes the act of platform riding look lovely.