I've put a bike on at Lille, two years ago. When we did it (five of us) my chum who did the organisation, had spaces booked. We rocked up at the station, handed the bikes over to the tender care of some railway employees and went to drink beer*. When the time came to get on the train, we spied our bikes in the care of the railway bods at the correct place on the platform so they could be hurled on to the train.
* we also watched a drunk man piss over the edge of the balcony, fortunately missing the OLE cables.