Hi, you can set a fixed ID to a segment using the Material operator: just set the "From" range to a same value.
For example, Replace Material ID 1, From 2 to 2, will replace ID 1 with 2. Sequence/Random mode has no effect.
Anyway this should not be necessary. Usually you assign the required IDs in the original geometry.
If you have several objects with their own IDs, and need to create the multisub, an easy way is to attach all objects in an unique mesh. Then, get the multi-sub created by Max, and extract the segments from the elements of the attached mesh. They must have the right's ID, according to the multisub.
Unfortunately it's not possible to assign several materials to a single RailClone object. This is a 3DS Max limitation.
I hope that helps.