The cast frames today are usually a zinc alloy. The metal dissipates heat so quickly that a run-of-the-mill soldering iron can't get it hot enough for solder to stick.
It's much easier to drill and tap a hole for a screw, and then solder to the screw head or tin the end of the wire and twist it around the screw and tighten with a small washer under the head of the screw to keep it from pushing the wire out..
Generally most locomotive frames are cast. This makes them very difficult to solder to effectively. Locomotives such as Athearn blue boxes and some RTR models uses the frame as one connection to the motor.
Pot Metal is a term used for a low temperature metal (low melting point) that can be heated up on a stove and have it change to its liquid form. Alot of times "pot metal" has a large amount of lead while most modern castings (in locomotives anyway) are using something like zymac (a zinc alloy) which has a much higher melting point.
The easiest method I have found to attach a wire to the frame for pickup is to drill and tap a hole and use a brass screw as something to solder to.
I generally drill and tap a 2-56 screw into the frame of the locomotive. I then simply screw in a brass 2-56 screw. I then tin the head of the screw with solder and then add my pickup wire.
Colorado Front Range Railroad: http://www.coloradofrontrangerr.com/