Escalators: Are they allowed to be used on Shabbos or not?
(and the reasons behind it)
R' Yehoshua Neuwirth, in Shemirath Shabbath in 23:52 permits the use of escalators. R' Chaim Jachter indicates (1) that this is because escalators don't have the problems with elevators expressed by R' Levi Yitzchak Halperin, the main authority behind prohibiting them.
For a detailed discussion of the issues with elevators, read R' Jachter's four-part series on the topic. The topic is complicated, involving a great deal of engineering and Halachic understanding, but it seems from R' Jachter's summary that the main problem with elevators is that the weight of passengers on a descending elevator assists the motors in making it descend. I'm not completely sure why this issue doesn't apply to "down escalators," but I suspect that passengers on them impede the escalator by increasing frictional force opposite the direction of travel at least as much as they speed it up by adding gravitational force partially in the direction of travel.
(1) Probably based on a footnote in the original Hebrew version of the book, which I don't happen to have handy. Anyone who does and wants to fill in details would be welcome.
Regular Escalators that don't have sensors which start working when one gets near them are okay Meikar Hadin but should be avoided unless one is Zaken or Holeh according to Hacham Yishak (Yalkut Yosef).
We already know of the use of Shabbat elevators is considered by many to be halachically okay. Thus, using an escalator would fall under the same territory. So long as the rider doesn't have control to change the direction or speed of the mechanical device or otherwise force a change in the power used, no problem.
On a less halachic note, the ideal of Shabbat is of "rest", a deep subject I won't broach here. The use of an escalator occurs generally in malls (at least in the U.S.), where you can attend on Shabbat but for what purpose? To say it goes against the "spirit" of Shabbat is too strong, especially because of the argument 'what if we build an apartment building with only escalators to be Shabbat friendly?'
TL;DR if a Shabbat elevator is kosher, so is an escalator.