Horse Power and Tractive effort

I know one is a force the other a Power, but ehat do they refer to and actually mean in terms of a locomotive?
Thanks for any help!

See if this helps ... http://www.alkrug.vcn.com/rrfacts/hp_te.htm

Al's explaination of TE and HP is great. I just want to add definitions of 3 terms that are often related and confused.
Force - The amount of push or pull measured in pounds or newtons (1 lb = 4.448 n).
Energy (or work) - A force acting thru a distance measured in foot-pounds or newton-meters (1 ft-lb = 1.356 Nm).
Power - The rate of doing work or of expending energy measured in horsepower or watts (1 hp = 746 watts).

Bob Weber

Very informative article. Shows one that there is a lot more to think about than just hitting the throttle and going down the track!

Thanks great help