Transfer Phenomenon: Why Coal Slows & Speeds Trains

[KishoreAM]

Imagine two railroad trains traveling on parallel tracks at different speeds, and workers shoveling coal from one train to the other. On the average, the impact of particles of coal going from the slower to the faster train will tend to slow down the faster train, and similarly the coal going from the faster to the
slower train will Lend to speed up the latter.
Why?
This question seems pretty obvious for somebody, but please try to explain it.

 

[BvU]

Helli Kishore,

In the absence of friction (and assuming the coal is thrown sideways wrt the faster train) you can split this up in two parts:

• a kind of collision process for the coal that lands on the slower train (and indeed, with momentum conservation speeding up the slower train).
• the separation (throwing it off) of the coal from the faster train does not influence its speed, though. Momentum conservation is still valid.