wese to wapsi ka safar bhi ap wese hi kersaktay ho jaisay ap Abbotabad jaogay.. so what's confusion here?
any way, if you want to travel by train, best way is to get coaster from Abbotabad to Pindi and take train from Pindi.. you can either get booking in economy class or AC business depending on your budget..
but if you want to opt to travel via bus.. you should be able to find bus from Abbottabad directly to Karachi... in bus no compromise less than AC bus but then obviously ticket cost will be higher..