The Plarail Land series of Plarail sets aimed at a younger age demographic than traditional Plarail (advertised as for 1 to 4 year olds in the 1970s and 2 year olds in the early 90s Disney licensed rereleases) actually saw seemingly moderate success in the international market, with multiple regions getting several of the sets exported and expanded, most of which are still fairly easy to find for sale used. These sets used larger, chunkier engines and focused around the gimmick of moving rolling "friends" - plastic figures with ball bearings in the base to allow them to roll down ramps, around playset features, and onto and off of the motorized vehicles. There are two basic types of chassis, the "bus" chassis which is front-wheel direct-drive that was also used in the zoo set and the "train" chassis that was used in the Ferris wheel and helicopter type sets.
The bus chassis is used in the 1977 Ponkikki Kindergarten Bus set and its rereleases and exports. In 1977 the set was sold under the Playrail range as Merry-Go-School Bus. The red chassis shown below is from a 1990 Mickey Mouse Japanese rerelease. The bus chassis was adapted for use in the Merry-Go Zoo themed sets. Five screws in the bottom of the chassis hold the bottom plate on. The sixth recessed screw with collar holds the sprung platform into the chassis.
I think this is actually a very nice gearbox. Not only is it direct-drive when pretty much no other Plarail vehicle was at the time but it is also front-wheel drive which is rarely seen. Four reductions nestle very nicely across two shafts including the driven axle by using several double gears that are free-spinning on their shafts, allowing for the gearbox to be nice and compact. I have not actually had any problem with any of the three bus chassis that I have - because the gears are fairly large except for the metal motor pinion and half of them are free-spinning anyways, they are unlikely to contract and crack. The gear ratio is just low enough as to where it can be backdriven with a firm grip on the wheels, allowing it to be jump-started by hand. This will probably eventually cause the static gear on the drive shaft to start slipping, so its better to take it apart and spin the motor by hand if needed.
There is a later Tomy "Big Fun" branded Merry Go Zoo that was made in China that may have some gearbox variations, but I do not have a copy to check. My guess would be that it is probably fairly similar but with a plastic motor pinion instead of metal.
The train chassis is used for the 1977 Ferris wheel sets and the 1978 helicopter sets. Japanese sets and the American Playrail Merry-Go-Train use a Japanese-made variant of the chassis while later export sets including the 1978 American Playrail Merry-Go-Copter were produced in Taiwan, with some other production changes along the way. Four screws hold the bottom of the chassis in place. Like other rim-drive gearboxes, they can be backdriven by turning the output spools, which can help jump-start an oxidized or seized motor.
The Japan-made version of this chassis has a rectangular weight and a rubber friction-drive system. The motor pinion is metal and the small gear in the second reduction on the same shaft as the contrate gear is an eight tooth gear that will probably be split.
The rim-drive tires on these engines are the rubber style in use at the time on other old power gearboxes. The striker for the clicking "chuffing" noisemaker is metal. The large gear on this shaft may also be split, but it can usually be glued back into place because the split does not usually reach the teeth of the gear.
For the 1978 helicopter sets like the Merry-Go-Copter production was moved to Taiwan and some slightly different toolings were used. Early Taiwan production engines like those in early American Merry-Go-Copter sets as well as Taiwan-made Palitoy Merry-Go-Train sets used rubber drive tires but later Taiwan engines like the pictured Tivoli engine for European export as well as later issues of the American Merry-Go-Copter used plastic rim-drive spools similar to those used in the set destination's bases to pick up power from the train's wheels.
Earlier Taiwan gearboxes still have the metal motor pinion and rubber rim-drive tires. The rear weight was changed to a cylindrical one and the clicker striker was replaced with a molded plastic cam similar in shape to the older metal type. The final gear on the same shaft as the striker has been changed to a double-type even though the smaller gear is not used, possibly so the same tooling for the gear in the previous stage could be reused. Both the plastic striker and occasionally the larger gears will split, although they can usually be glued back into place as the split does not always reach out to the actual teeth of the gear, although if these gears continue to contract with age they may need to be replaced anyways. These gearboxes use a 9 tooth gear instead of an 8 tooth gear on the first gearbox shaft which is usually split. Later Taiwan production gearboxes have all plastic gears as well as the plastic rim-drive spools, which are somewhat nicer because they do not perish but I suspect they provide less traction than a nice, fresh rubber tire would have. The plastic motor pinion is an 8 tooth type and there is a chance it will also be split.
A later Taiwan revision of the train chassis adds protruding guards down around the sides of the rim-drive spools. I have only seen this style of chassis used on later European Tivoli trains, with my examples being the blue-chassied Tivoli Train. Like the previous Taiwan gearbox, this chassis uses plastic rim-drive spools and mostly plastic gears, although interestingly the motor pinion and small 9 tooth gear are both metal.
The motors in most of my engines squealed loudly when first run after many years, which is usually the result of dried lubrication inside the motor. The best long-term solution would be to open the motor up, clean and regrease the motor brushes and commutator, and lubricate where the shaft spins in the motor housing, or indeed to replace it altogether with a brand-new motor, although I generally like to leave original motors unless they are non-functional.