A minecart was a type of small railway wagon that had no engine, and was entirely propelled by human power. Some types of them were used for holding coal, ore, diamonds and whatever else that was mined.
Superman was lured into a trap by Doctor Hiram Gulliver, who made him think that the Dynamic Duo was in peril, as he saw them tied to a minecart rolling down an inclined railroad. It turns out it wasn't Batman and Robin, but just dummies. Nevertheless, Superman falls for the trick and he is shot by Doctor Gulliver's handheld minimizer.