I see these every time a bicycle path intersects with any kind of car or train infrastructure. It just seems really patronizing, as if we're not capable of slowing down without getting off the bike; it also shifts responsibility onto the cyclists if there is a crash based on something arbitrary (mounted vs dismounted).
The railroad ones almost make sense depending on the conditions. At a level crossing which goes across the road at an angle rather than dead perpendicular, the groove next to the rails will catch your front wheel which may or may not result in you getting thrown off and/or winding up with a bent rim.
It's not a problem if you're paying attention. Assuming that most bike riders are paying more attention as compared to, say, most car drivers is probably still being unreasonably charitable. But in that case they're unlikely to obey a sign, either.
The ones we have here spell it out with "angled rail catches wheels" and a picture of a ball headed safety dude getting bucked off of his bike rather than just blithely telling people to dismount.