This is a good model of an RS3. The single motor drive places the whole unit low on the rails so it looks more realistic than the later dual motor drives where the whole thing sits too high does not look right compared to photos. The hand rails are made of engineering plastic, delrin is one of the name brands, They are plenty sturdy enough for normal handling of models. This is not a kids toy, but is not super fragile either. If want even stronger stuff, by brass castings from P&D or some other shop for the handrail stantions and replace the railings with wire.
All that said a lot of early units had failed components in the drive train. I was told by an plastics engineer that the supplier was probably running the molds for the gears too cold and they were melting together when poured, and hence cracked. I have many of these, after the first failures, they have generally been very reliable.
Now it sounds like your problem might be the top sprocket on the motor shaft that is cracked. The drive consists of a top sprocket on the motor connecting to a lower sprocket/shaft via a chain. The lower shaft than connects to each truck via sliding universal joints. Each axle is powered with a worm gear from the shaft onto the axle gear. All the components are easy to replace if they break. It is a great design to be self-maintained.
There could be other problems, but I would start with that one because you described the hum, which implies the motor is turning but not the mech. If it were axle gears, the unit would run, but be jerky.
To take it apart, there are six screws on the chassis underneath. There are 2 on each end, and 2 hidden above the fuel tank. The tank sort of snaps onto the frame. One end has small tabs facing out from the center. You gently squeeze at that end and lift the tank away, and then slikde the other end out slots facing toward the end. Then you can see the bottom of the "gear" tower.
Also before you lift the shell off, gently remove the hand rails from the little holes in cab.
ONce the hood/cab is off you can test run it again and you can tell if the motor is moving, but not the sprocket.
As mentioned above P&D Hobbies has replacement parts for nearly everything in this drive. They have had new runs of the various gears and sprockets made. They also have new axles with gears if needed. It is very hard to pull the wheels on the 3 rail axles to replace gears. There are 2 lower sprockets, a "small" one and a "large" one. The large one is usually used in 3 rail units to get the desired higher speeds.
A word of caution, the body mount screws are metal on plastic. Remove and install them by gentle hand work, don't use power tools. If you use a powered driver, you will most likely tear up the threads in the plastic pars.
