The following are some of the items that are appropriate:
The number of engines
The number of cars the engines are pulling
The total tonnage being pulled
The speed at which the train is moving
The total length of the train
Fuel consumption per mile of travel
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you would measure it with feet
What is the most appropriate instrument to measure
A centimetre.
inches
The appropriate unit to measure weight is the newton. Now, if it is really the mass you wanted, you could use kilogram or gram.