How to help the subway to save electric energy? You get in the middle of a subway car. The subway accelerates, coasts, decelerates, optionally recouping energy.
Is there anything you can do to make the subway use less energy? Walk or roll forward or backwards during acceleration, coast or deceleration?
Rolling on a bike backwards during acceleration with the generator  and light on will extract energy, so that seems wrong. Walking is similar (it's like walking down a mountain, which will convert potential energy to heat).
Would walking forward during acceleration help? Or changing position during the coast phase?
I think walking back while decelerating with energy recouperation will give the subway extra energy.
 A: Yes you can. It might even be practical for very specific constraints. Essentially you have to become a kinetic energy storage device that is able to extract kinetic energy from the train during the braking phase and return it to the train during the restart/acceleration phase. You have to become a flywheel. 
Now I assume you do not want to be strapped to a rotating mechanical device that is part of the drivetrain, so you will become the linear equivalent of a flywheel. 
Step 1
During the braking phase, you 1) run or 2) roll on your bike from the back of the train towards the front. Because the train is decelerating with respect to the track, you will experience a forward acceleration which helps you accumulate velocity/KE inside the train. 
In option 1) you will actually assist in braking if you can achieve a sprint acceleration higher than the train's braking acceleration. In option 2) you are simply maintaining your personal kinetic energy with respect to the track even as the train loses it's kinetic energy from braking. Remember...flywheel....
Step 2
Do not run out of room before the train stops. If you have to stop sprinting or rolling, the experiment ends and you achieved nothing. The moment you begin to slow down, your deceleration in the train will cause the train to accelerate (and the train will just bleed off that accumulated KE in the form of heat after having to apply more brake). We need a really long train.
Step 3
Hopefully before you reach the end of the really really really long train, it has come to full stop and is time for it to depart again. As it begins to accelerate from the track, stop running/rolling. Your accumulated kinetic energy will hopefully and mostly be trasferred back to the train and assist in the acceleration.
Step 4 
Congratulations, you have achieved linear flywheel-hood by assisting both braking and acceleration of the train to reduce fuel costs. You might end up being hungry though so probably not worth the trouble.
