Creep = the movement of the rail through the fastenings.
We like to know where the rail is moving to so that we can control the stress in the rail. The term 'stress' is not a really good description as its more like controlling the tension in the rail so it can handle the range of temperature and the expansion and contraction of the steel. In the old days you could monitor your creep with observations of the joints etc. With continuously welded rail, if you end up with too much rail in one place you are suseptible to heat buckles and also alternatively too tension in cold weather with a higher propensity to have broken rails ( the tension acts on the defects in the rail).
What makes the rail move?
There are three kinds of creep.
1. Grade creep - where the rail hangs off the hill and heads for the low points.
2. Tonnage creep - where the rail moves in the direction of the tonnage
3. Steering creep - a bit rarer but due to the steering aaction of the wheels pushing one rail ahead of the other. A good example of this was on the Pimba bank I recall.
One aspect of the three kinds of creep is that they can co exist together or be any combination etc.
Hope that helps