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Early electric trams used single powered cars, as those required a lot less elaborate electrics compared to articulated or even remote controlled multiple units. To expend the capacity, unpowered trailers were added.
Also makes sense as many systems were converted from horse or steam-powered and had trailers to spare anyway.
While these were bidirectional on their own, it required shunting at the end of a line, so they started building loops. And when articulated trams became common, they were of course built fully unidirectional to save the second cab and half the doors.
