elevate the 12-volt system away from the more limited 4.5-volt system. Notable train sets include the Electric Intercity (7740) and Steam Locomotive (7750), particularly for its large wheel pieces which were not found in any other set. 7750 and the 7727 Goods Train also featured a red version of the 12V motor module that also has become a very rare and sought after component. Owners with a worn out red motor had no option but to replace it with a standard black one (7865) or break open the sealed unit and transplant the components. The 12-volt line was promoted in a 1983 UK television advert featuring a group of adults planning a 'mail train robbery'. No new sets for the 12-volt system were introduced after 1986, although the full range remained on sale leading up to its eventual replacement in 1991. Lego continued to supply accessories (but not complete sets) for the 12V system via its mail order service until 1993. The Nine-Volt Era (1991-2007) 1991 saw again a major overhaul in the train line, in line with the ongoing transition to the new generation 9-volt system that had first appeared in 1986 with the 'Light & Sound' line of sets, with Technic having made the transition to 9-volt the year before. Tracks gained a new appearance with power being transmitted directly through metal strips on the two running rails. The new line abandoned the 12-volt power in favor of the 9-volt system. One of the key weaknesses of the old 4.5V/12V system was the unreliability of the pinned connector plugs, which were notorious for becoming loose or for the wires to fracture over time, whilst the 9-volt system used more positive fitting contacts integrated into the brick studs. The remote-controlled accessories from the 12-volt system however were also abandoned, with only manual point control available and no signaling capabilities. At the same time, the 9-volt train motor was made train-specific by its fixed wheels, while the similarly sized 9-volt universal motor changed from individual metal axles wheel holes to axle holes for Technic axles, for which there were no train wheels available. Previous 12-volt locomotives were not compatible with the new system without modification, due to the change in voltage and means of powering the motor, although it was possible to retrofit them with 9-volt motors. The gauge did not change, therefore older rolling stock could still be used.