1990 – Iron Mule Train – Revisited by D.J. Todd (American)

Todd's paper is very readable. See pdf here. Note that although referred to as a "train", Todd appears to have only built one. So the original "train" aspects have been lost.  Its interesting to note that "BigDog" does have the concept for a "train", where the lead dog follows its human master, and other 'dogs' …

1968 – Iron Mule Train – R. A. Morrison (American)

Although referred to as the "Iron Mule Train", only one carriage was built for test purposes. I believe it was Frank Tinsley's original idea in extending Space General's original Moonwalker that inspired Morrison into promoting this third variant (the other being the Moonwalker disability walker after NASA's cancellation of the earlier unmanned moon rover programme.)  …

1961 – Iron Mule Train (Concept) – Frank Tinsley (American)

Although published in 1962, the illustration is dated 1961. Frank Tinsley's concept pre-dates A. J. Morrison's "Iron Mule Train" by about 7 years. Given the cancelled Space programme for an un-manned lunar misssion, I wouldn't be surprised that Tinsley's concept inspired Morrison in resurrecting the Moonwalker project (having already had a second life as a Disability walker) …

1961 – Moonwalker Lunar Rover – Space General Corp. (American)

Space General designers have built an insect-like vehicle with six legs, two arms, a triangular body, a solar-cell panel, and an antenna. The left arm, ending in a claw, picks up objects to examine. The right one holds a TV camera to do the looking – and to see where the vehicle is walking. This …