City planners adopted railroad technology in the Industrial Age to help out commuters. Since a lot of factories were not located close to homes, people took the train to get to work as their way of transportation.
Around the time of Julius Caesar the form of roman government effected the change from the form of Republic to that of Empire. This, however, only happenned after his death by assassination and was succeeded by his grandnephew Octavius, who in fact was the first Roman Emperor.
The immediate effects are easy. Miles of city would be leveled and all inhabitants of that area would be almost instantly killed. The long-term effects would be there would be so much radiation your body could go through nasty mutation and you'd need to be treated if you survived but if don't survive you'd have a long agonizing death. Also all neighboring cities would have radiation surging through them.
