The photo shows one of the three Thames Trader-based buses used by the Dartford Tunnel Joint Committee to convey cyclists and their mounts through the tunnel. This was a free on-demand service and there were special docking stations at each end of the tunnel (then single bore). The buses were operated and maintained by LT but owned by the DTJC (Essex & Kent CCs).

Estimates of the number of cyclists using the tunnel (pre-M25 of course) were hopelessly optimistic and the buses saw little use. They were withdrawn and cycles henceforth carried on trailers pulled by the Land-Rovers normally used to escort fuel tankers through the tunnel.

I believe one of the Thames buses (TT-class) is owned/preserved by a certain Managing Director - Surface Transport, TfL.

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Another short-lived service was the LT Country Area route 399 (Dartford-Grays). Numbering this obviously caused head-scratching at 55 Broadway in view of the convention that country routes north of the river were (mostly) numbered in the 3xx series and south of the river in the 4xx series.

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The longest-lived and best known route through the tunnel was Dr Heffernan's Thames Weald service between Romford and Sevenoaks (and variations).