The actual slump angle is a matter of many factors - the geology, the flora, the weather conditions, etc - this may be of some interest - https://www.newcivilengineer.com/latest/slope-engineering-uphill-rail-challenge-in-somerset/8661241.article - these days, they often 'net' slopes that worry people, even if they've been there for a long time already.
A glance at local examples would be a good guide, in general.