This is a limitation with all interactive renders, including IPR:
RailClone generate and keeps on memory a complex set of data, including instances and custom meshes.
Usually data for viewport is different from render, due to the Display setttings.
But when using IPR, there are multiple render and display threads running simultaneously, requesting each one their own data. For this reason, we disable the viewport visualization.
The only solution for this, is to generate two separate data sets (one for viewport, and other for render). This is the way that Forest Pack works.
We tried same approach with RailClone, but it caused a lot of side effects, because code is more complex that Forest. Besides this technique have a significative performance hit (object must be rebuilt two times).
To make things even worse, there is not a standard defined in 3DS Max for interactive renders. Each render engine (V-Ray, Corona, etc.) use their own method, which force us to implement multiple workarounds in the code.