LEGO Hidden Side Sets

the trains in the 90s don't use batteries.  They use 9V transformer.  I found the following information on Wikipedia.

"1991 saw again a major overhaul in the train line. 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 to make it compatible with the battery-operated elements found in the Lego 'Light & Sound' line of sets. The remote-controlled accessories from the 12-volt system 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.
The continuing availability of the train system was thrown into doubt in 2006, with the release of the first 'remote control' train sets, which used battery-powered motors and did not have metal conducting strips in the tracks. For about a year, both systems were available, with the 9-volt system being marketed under a "Hobby Train" brand, available direct from Lego. By the end of 2007, the 9-volt system had been discontinued."

I attached a photo of what a motor truck looks like.