Hi,
I'm afraid it's not possible to get that information from Maxscript.
In this case you're using RailClone as an instancer, where each item corresponds with an instance of the source object.
But we cannot guarantee this happens always. Usually an input segment may be sliced, deformed or transformed, generating different meshes. So, the output of RailClone is a collection of unique meshes and instanced segments.
For this reason doesn't exist a Maxscript interface for this. Instead, in Forest that is possible, because all output items are instances of the source objects.