We never had any problems with our tandem, other than that it could be tight on busy trains, and that not all lifts on stations were big enough. We also used a trailer with the tandem, and although officially banned, it was never an issue. But be realistic: if the train is full it is full, and be quick to get on and off. Dutch trains run on very tight schedules, and have only some 3 minutes at the station (sometimes less). So play your part to make sure the train is not delayed.
Willem