The bus company would only be liable, or financially responsible, if they were at fault in some way, such as, for example, if you feel because a step or guard rail was loose; or you fell because the trolley was going too fast for conditions. If the bus company and its driver did nothing wrong that caused you to fall--if it your own clumsiness, for example, or simply an accident with on one at fault--they would not be liable and there would be no point in suing them. If you think that they may be at fault, consult with an attorney about the situation; the lawyer can guide you about where to file the case.