In the Eurometropolis Lille-Kortrijk-Tournai, there have been proposals to extend the Lille metro across the border into Belgium. Although those proposals have been shelved for cost reasons, I wonder: does any binational metropolitan area have a mass rapid transit system extending into both countries?

NB: I'm specifically thinking of metro/underground/subway/lightrail/tram, which are inherent to urban areas. There are many international bus systems both in cities and rural areas, but buses do not need any particular infrastructure.