That may be the case because Polar Express is made using [motion capture][1]motion capture Technique, Which is not used in Shrek, Toy story etc. Motion capture techniques are generally used to present real emotions or live movements. Or may be the case that its animation is not made that playful as Toy Story or Shrek to make it more realistic. In general, Polar Express got good appreciation with mixed reviews but it received a cult following.
So it made this way on purpose and I think it's done to make it feel realistic. [1]: http://en.wikipedia.org/wiki/Motion_capture