I have following
1- 3d rail sting with chainage/station
2- platform height and offset from this 3d rail string
how do I generate a 3d platform string using the above
In plain MicroStation, I think I would try "Offset Curve" to get the horizontal offset, them "Move" to get the vertical offset.
btw: You might get better answers if you post to the MicroStation forum (or the Geopack, Inroads... forum if you have one of these products).