A typical railroad car can carry about 3 times its own weight. The empty weight of a 4-axle car is therefore about equal to the maximum axle loading, which is usually 33 tons in America. A rule of thumb is that the drawbar force needed to move a car on level track is equivalent to a .25-percent grade, or 1/400 of the car's weight. That works out to 165 pounds.
The force to start the car was an issue with the now-forbidden friction bearings (which gave rise to the technique of "bunching the slack" to start a train); but it is not so important with modern roller bearings.
Notice that the way those strings are angled upward from the toy-train's coupler, they would lift the tender off the track before any serious pulling could happen.