Two men from Yemen who trained with al-Qaida to fight US forces helped an American they befriended join the terror group, clearing the way for him to hatch a plot against one of the busiest US commuter railroads, prosecutors said Tuesday. Saddiq Al-Abbadi and Ali Alvi were captured in Saudi Arabia before being brought to the United States to face charges of conspiracy to kill Americans and providing material support to al-Qaida. Alvi, 30, was to appear Tuesday in federal court in Brooklyn. Al-Abbadi, 36, had an appearance Sunday. The names of their lawyers weren’t immediately available.