Rail charges were small explosives used as ammunition in Rail Detonators. In addition to being powerful explosives, they stuck onto the point of impact for a few seconds before exploding, thus possibly taking out more targets than originally intended.
In terms of explosive force, this weapon was very powerful, with seven or eight rounds being able to destroy an AT-ST. One shot would take down a stormtrooper in full combat armor. It was also one of the few weapons, including the lightsaber, able to kill a full size Kell dragon.