I've got an old bar I use for stumping, demolition and tarry railroad ties. Crank up the bar oiler, and be prepared to sacrifice chains! I usually just use whatever old rocked chains I've got that are on their last legs, and send 'em to meet their maker coated in creosote. WD-40 will clean up the bar when your done...
The saw might be another story. Inboard clutches with spur sprockets seem to be best for this type of work, as they are much easier to clean. That tarry crap gets everywhere!
Also, watch out for embedded rocks and metal hardware... sometimes its better to cut both ends of a tie to get it to the right length, if it means you can find 'clean' wood to cut.
Make sure your chain tension is good, and your chain-catcher works!