tf::transformBroadcaster shows error of Lookup into the past, best practice for setting up broadcaster and listener

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Follow up with my previous question #q283013, currently I have another problem with tf broadcaster with same error of lookup into the past. I am really confused now and I hope that you can help me understand a proper way to set up tf broadcaster/listener. I followed an example here #q90246 and fixed my listener. Here is the code:

#include <...my stuffs..>
class cloudHandler{
public:
    cloudHandler():
    {
        main_sub = nh.subscribe("pico_flexx/points",1,&cloudHandler::mainCB,this);  
        rail_plane_pub = nh.advertise<sensor_msgs::PointCloud2>("rail_plane",1);
        fit_rails_sub = nh.subscribe("rail_plane",1,&cloudHandler::fit_railsCB,this);   
    }
    void mainCB(const sensor_msgs::PointCloud2ConstPtr& input){ 

        <...my stuff...>     
    }
    void rail_plane(pcl::PointCloud<pcl::PointXYZ>::Ptr input, pcl::PointCloud<pcl::PointXYZ>::Ptr cloud_projected){
        <...my stff...>
    }

    void fit_railsCB(const sensor_msgs::PointCloud2ConstPtr& input_from_camera_frame){              
        try{
            cam_bl.waitForTransform("base_link",input_from_camera_frame->header.frame_id,input_from_camera_frame->header.stamp,ros::Duration(2));
        }
        catch(tf::TransformException &ex){
            ROS_WARN("%s",ex.what());
        };

        sensor_msgs::PointCloud2 input_in_bl; 
    pcl_ros::transformPointCloud("base_link",*input_from_camera_frame,input_in_bl,cam_bl); // this is ok to transform now

    <...processing my stuff...>

    /*Now I tried to published my result*/
    tf::Vector3 rail_origin(p.x,p.y,p.z);
    static tf::TransformBroadcaster rail_br;
    tf::Transform rail_tf_;
    rail_tf_.setOrigin(rail_origin);
    rail_tf_.setRotation(q);
    rail_br.sendTransform(tf::StampedTransform(rail_tf_,ros::Time::now(),"base_link","rail")); /*Can't send broadcast here, the node keeps dieing and restarting which cause a broken tf*/

    geometry_msgs::PoseStamped r_p2;

    r_p2.header.stamp = ros::Time::now();
    r_p2.header.frame_id = "rail";

    r_p2.pose.position.x = p.x;
    r_p2.pose.position.y = p.y;
    r_p2.pose.position.z = p.z;

    r_p2.pose.orientation = tf::createQuaternionMsgFromYaw(yaw_rail);

    rail_pose_pub2.publish(r_p2);/*Can't even publish this message*/
  }
private:
  tf::TransformListener cam_bl;
  ros::NodeHandle nh;
  ros::Subscriber main_sub, fit_rails_sub;
  ros::Publisher  rail_pose_pub;  
};
int main(int argc, char **argv){
    ros::init(argc, argv, "pico_rails_node");
    cloudHandler handler;
    ros::spin();
    return 0;
}

I have no idea why I can't send my tf broadcast. I believe it is proper to stamp my broadcast with ros::Time::now() since it requires sometime to process all my stuff. I also tried to stamp my broadcaster with the time stamp of the incoming message but the error still happened.

[ERROR] [1518989417.344710428, 1518732057.962978755]: Lookup would require extrapolation into the past.  Requested time 1518732055.923351000 but the earliest data is at time 1518732056.005952182, when looking up transform from frame [pico_flexx_optical_frame] to frame [base_link]

Obviously, I have missed something about tf here. I understand that:

1. a tf listener requires some time to cache up all the transforms, so in callback, the first few calls should just return;

2. if we can look up transform, it means that the tf tree is set up properly, then a broadcaster can broadcast anytime with correct transformations between frame_ids. It is logical to stamp this broadcaster with a current time.

So my question is :

1) Could you show me what I misunderstand and/or miss here?

2) Is there a proper way set up the tf to avoid this situation? Example for my class, I put every publishers and subscribers in class constructor, and all callbacks are for ... (more)