I’m always interested in Singaporean English neologisms that are used in formal settings, for example the use of “handphone” where others might use “mobile phone” or “cellphone”. In this case, “up-riding escalator” means pretty much “this escalator goes up” (i.e. the sign’s saying don’t be stupid and step on the escalator from this end).
Another example of the use of “up-riding” can be seen in this Google cache of a Today newspaper article (“Although there is an up-riding escalator, users have to descend by the stairs at the SGH end.”).