Unfortunately Redshift is not support as a native render engine in RailClone, so by default it creates a huge mesh with all the geometry, triggering the limits error.
The only way to render larger objects is using RailClone Tools, instantiating all segments to stand-alone instances.
I've done a quick test using a Redshift proxy, and it seems to work correctly. It creates for each segment a Max instanced node, which keeps the original proxy. Render is fine as well.
Please would you detail about that Redshift doesn't instance 1:1 ?
In any case, we encourage you to write to the Redshift team, requesting they add support for RailClone.
The more customers request it, the more they will be willing to implement it. We already sent to them the necessary tools, and of course we'll be happy to help if needed.