Originally Posted by mm43
My 3 year old grandson proved it to me when he set up two parallel lines of Thomas wooden railway and placed one engine and a carriage on one line, then one engine and a wagon loaded with sand on the other. He pushed them off together, and you know the answer - he observed it also.
Let's study that in some detail. Presumably the speeds achieved by your 3 year old grandson were low enough for the aerodynamic drag to be insignificant. So the retarding force is the result of the friction between the wheels and the track, and the friction in the wheel bearings. Normally one would expect those frictions to be proportional to the loads on the wheels, i.e. the masses of the two trains. In that case the two trains would slow down at the same rate. If that didn't happen, there must be other differences besides the difference in mass. It would be interesting to extend the experiment to find out what these differences were. If you loaded both trains to have the same mass, would they decelerate at the same rate?