The pictured and linked British Rail Class 121 is definitely categorized as a train, not a tram. If you look at their chassis alone, you can see how different the vehicles on those 2 pictures are. Trams are light and maneuverable, so they could make often tight turns riding (mostly) in the middle of town streets and also making stops very often... which you can easily see is not a case with Class 121. Also it's quite exceptional for a tram to be diesel powered.