The gondola method will not give you the right answer. If a locomotive can pull a 5 lbs car, that does not mean it has 5 lbs of pulling force. These two things are very different.
A rail car of a given mass presents some fraction of its weight as drag when acted upon by a force. Think about a disabled car in the roadway. A human can usually push it to the shoulder, but that does not mean the human has 1-2 tons of pulling power.
In addition, using weighted cars while adding weight to them as a comparison between locomotives is suspect, because the amount of “resistance” or drag produced by the axles may not scale linearly with the amount of added weight.
