Sorry I could not reply to your post promptly
I work everyday from 8 to 6 an sometimes I have to stay late at work. All apologies.
I think that a good hypothesis will be your idea on adding weight to the train instead of measuring height.
If you add weight to your train it will slow down, that is a fact. But how about if you build your hypothesis by stating that if you use a specific material in the frame of the train that will help it to gain speed.
For example, big trucks switch from steel to aluminum in the wheels because aluminum alloys are strong enough to do the task and also they are a lightweight material.
They gain speed and work efficiently because they save fuel…also.
So, how about if you try different materials, run your experiment and come up with some result. Build a chart to show your results and select a material as a winner after proving your hypothesis.
Hope this works for you.