My theory is that with the Railgun, the actual psychic power was, at the moment of impact to Touma's hand, still propelling the coin forward, so when Imagine Breaker stopped it, the coin no longer had any force, making the inertia null(ex: you push a box. As long as the force(in this case, you) push the box, it keeps moving. Once the force stops, the box doesn't move). Accelerator's power affects the object and its force is an after-effect of his power, which is why Touma can't stop them.
I still think it's impossible to react to the Railgun though, I don't know HOW he did that
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