Where did air come from? ANSWER: Well, how far back do we want to go? Could the pipes explode? ANSWER: This is a sure recipe for disaster. Do not do it. The wheels are not slipping and furthermore being steel are not very deformable, so their contribution to the friction is rather small and probably similar for the two trains. The average acceleration in your example is change in velocity divided by the time it took to to reverse the velocity. Since Christians struggle against sin their entire lives, this consequence of justification is also for them an obligation they must fulfill.