A train suddenly stopping would've caused at least some serious injuries among the passengers.
A single wooden plank would never be able to absorb even a tiny fraction of the kinetic energy of a train traveling at speed without shattering, and even if it could, all the people inside would still retain their inertia, and go hurtling forward into the front wall of a suddenly-not-moving train car.
Alice says she has planted a bomb in the school, and says that it is C4. However, when we are shown the bomb, it clearly consists of several bottles of liquid. C4 is a plastic explosive, not a liquid.
Trains have a fail safe air braking system. If the brakes fail, the air comes out of the brake pipe, causing the brakes to apply and stop the train. It does not cause a runaway train.
After Batwoman stops the train, her grapple line detaches from the railway sleeper to which it's attached and comes hurtling towards Batwoman. This is impossible as it has no momentum nor does it undergo any change in loading that could impart an impulse to it and thus accelerate it as shown. The only way this scenario could happen is if it came unstuck at the moment the grapple line went taut (i.e. when the train is initially stopped), but it would not in reality come unstuck some moments later as it does in the show.