Ah, there’s a Wikipedia article on Lego trains, and indeed it is very helpful for understanding the broader picture of the history of Lego trains. What I saw happening with the 9 volt series Lego train components was just the tip of the ice berg. So, let take what I read about the bigger picture history and explain it from my perspective.
The earlier Lego trains were designed to be very similar to the established model train systems. Matter of fact, Lego train tracks are only slightly wider than the model train O-gauge rails, probaly mainly due to the need to make the tracks an integer number of studs wide. The earliest Lego train motors in 1966 were battery powered. Later on in 1969, 12-volt rail-powered motors came into the scene. Around this time, Lego trains had fairly sophisticated remote track control features, similar to those found in the established model train system.
But then, in 1991, when the 9-volt Lego train system came along, the Lego trains line saw a slowdown in sophistication. The remote control panels for track switching and multiple train control were done away with. The motivation for moving to 9-volt motors was mainly done so that the same 9-volt electrical components used in the “Lego Light & Sound” line of sets. So, there you go, now you know that the accessory voltage supply line coming off of the Lego train motor is 9 volts. Also, probably by deliberate design, Lego Mindstorms motors are also 9 volts and use the same Lego power cables. In 2007, the 9-volt train system was discontinued due to a desire to use cheaper pure plastic train tracks, a cited lack of volume sales, and issues with the mass production machinery specific to 9-volt train parts that would need major servicing.
In October 2006, a line of battery-powered train motors was introduced. In 2009, this system was changed to use the new “Power Functions” systems from the Lego Technic line. Again, the long-standing motivation here is the same, to merge the electrical functions of Lego trains to share more in common with other Lego product lines. Also, the desire to make the tracks cheaper have driven the metal-railed conducting tracks out of the business, and they are showing no signs of coming back.
Aside. What does Wikipedia have to say about the current financial status of Lego? Nothing! Well, we know from other sources that Lego is undergoing tough times from changes in the toy industry and new forms of competition. Indeed, this knowledge is instrumental in understanding many of the discontinuations that have been ongoing in the Lego Trains line.
20181212/https://en.wikipedia.org/wiki/Lego
Also, worthy of note is Wikipedia’s mention of the precursor of Lego, Kiddicraft self-locking bricks. Ole Kirk Christiansen specifically saw this toy first before bringing Lego to market, but unfortunately has been forgotten in many historical narratives. Alas, we must admit that now, even Lego is being forgotten from the larger historical narrative in some ways.
20181212/https://en.wikipedia.org/wiki/Hilary_Fisher_Page#Kiddicraft
Legos are traditionally made out of ABS plastic, and that material has not changed in many decades.