"MIG" wrote in message

This morning I've seen a three-unit train running on the DLR, with the
designation "Special", in between other services.

(Am I the only one having trouble perceiving the "articulated
vehicles" as anything other than units of two coaches?)

Like any other articulated vehicles (particularly trams), the short
coaches share a bogey. The only mainline trains with this configuration
in the UK are Eurostars, but they're quite common elsewhere.