The "Monorail Transport System", Lego Item Number 6990, was a LEGO set released in 1987. It was themed around LEGO's Space theme, one of three themes LEGO had at the time. As the name suggested, it was a monorail. A working monorail, powered by a 9-volt battery. It had several feet of track, 715 pieces, and five minifigs. The aesthetic was, of course, futuristic, as it was understood in 1987. Lots of black and white, with big geometric translucent blue panels. It took a long time to put the base and train and vehicles together.
LEGO had been dealing with train sets for a while, for over two decades, but this was the first time that LEGO crossed over the idea of a train set with their themed "LEGOLand" sets. The result was an expensive and complicated set that would captivate any young (or old) LEGO collector. This was the type of present you would get for Christmas if you were very lucky (like I was). After putting it together, there was one problem---the replay value of the train going around in circles was not as great as it could be, but luckily, with the basic premise of LEGO, to mix and match, the train could be rebuilt to belong in any number of scenarios, in the Space theme, or you could anachronistically throw it into the Castle theme and have your knights tilt at monorails.
It was a great and innovative idea by LEGO, but it did presage a trend that would increase very quickly: LEGO sets became less modular and general purpose, and started including more specialized pieces that it was hard to make general creations from. But at the time, the Monorail Transport System provided the cornerstone of my play for months, and I was probably not alone in that.