This increases resistance to rolling which needs to be made up by the locomotive.

You don't get free energy by sapping it off something else. It's pretty much a zero-sum game, or actually a negative-sum game based on energy lost as heat production.)

I agree with you to the extent where the attached system starts exploiting the energy from the movement of the train.

But in my opinion, the pistons are pretty small but in a chain of thousands. At one particalar time only one small piston is pressed by each wheel. Again its a matter of <hit and trial> to find out the amount of pressure we can extract from each wheel(12 tonns of dead weight) without effecting the overall performance of its energy consumption.

Small=inefficent.

There is no free energy from a moving tain. It takes energy to keep it moving, and the pistions will slow it down and require further energy consumption to overcome the pistons speed loss. Not to mention the depression of the pistons won't create enough are movement to effectively capture worthwhile enegy.

I'd put the energy from rail to be, it just won't work, as physics won't let it.