onGetViewFactory only called once for multiple widgets

Based on the above description another trick you can do if you are setting intent extra other than string could be:

Random random = new Random();
intent.setType(String.valueOf(random.nextInt(1000)));

This will invoke a call to onGetViewFactory();

I haven't tested this myself, but looking at the source code for RemoteViewsService.onBind(), I believe you can not just vary the extras in your Intent in order for it to detect that a new call to your onGetViewFactory() method is needed since it uses the Intent.filterEquals() method for the comparison:

Determine if two intents are the same for the purposes of intent resolution (filtering). That is, if their action, data, type, class, and categories are the same. This does not compare any extra data included in the intents.

I would suggest passing the information that you need (widget id?) through the Intent's data instead, maybe something like:

private static void fillInList(RemoteViews widget, Context context, int appWidgetId) {
    Intent intent = new Intent(context, AppWidgetService.class);
    intent.setData(Uri.fromParts("content", String.valueOf(appWidgetId), null));
    widget.setRemoteAdapter(R.id.appwidget_listview, intent);
}

and respectively on the receiving side:

public TreinVerkeerAppWidgetListFactory(Context context, Intent intent) {
    this.context = context;
    appWidgetId = Integer.valueOf(intent.getData().getSchemeSpecificPart());

    System.out.println("New TVAPWLF for: " + appWidgetId);
    trains = WidgetData.getInstance().widgetMap.get(appWidgetId);
}