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OK, this is very easy. No XPresso I promise. This is a step-by-step guide because there are *ahem* issues when uploading scene files... Create your Rail Spline. When I test things like that I want to be sure it works for all possibilities so I chose an irregular Spline ... Sine Wave Made it editable so I could elongate it Now put your guiding Null on a Cloner. Object Mode. Assign the Rail-spline to it. Make only two instances of the Null - we need only two guide nulls. Adjust the End to bring the 2nd instance closer to the 1st. The distance does not matter we'll fix it later. Make a Tracer and put the Cloner in it. Set it to Connect All Objects. Now create the object to be sliding... I made this shit Make sure you position its Pivot Point to the end/edge of it. Make a new Cloner. Set that to Object Mode. Assign the Tracer to it. Use one one instance. Now adjust the End of the first Cloner to match the other end of the object as close as possible... Mine was at 11.5% Now if you animate the offset you get what you need But if you want multiple objects to slide that's a different method, but shorter. You'll need these ingredients A Cloner A Target Effector A Geometry Axis Node A Geometry Orientation Node Set the Target Effector to Next Node and Use Pitch. The Cloner is as usual to Object mode and fed with the Rail This time I used just Cubes You just need to adjust the End to regulate the distance between instatnces. Yours won't look perfect with the first try, mine did not either. That's why I used the two nodes. The first was to adjust the Pivot of the Parametric Cube (you might not need it cause it's a custom mesh and you can move the Pivot wherever) The second one was to properly align the orientation of My cube along the Spline as nothing else could fix it. And there you have your train You can use a copy of your spline to make the instances appear as if they are always just touching the "now-invisible" Rail or use a Plain Effector to Offset them a bit higher ...

Rigid-body dynamics. I've updated my previous post for clarity.

Wow. Ask and you shall receive. Thank you Cerbera. I kind of knew some of those (like the fog and booleans) but the others I will investigate further. It's like working on your house-pull away the sheetrock to find all sorts of surprises - some fantastic and some mind boggling (but all in the good way).

Volume builder is just another set of tools in the toolbox, but particularly suited to a number of specific situations. Modelling is only a part of what it does. Pre R2025.1, it found a lot of use in making up for the deficits we had in the boolean department, but that aspect has become less important since the new Boolean Tool arrived, which in and of itself solves a lot of problems and annoyances the old one had. However VB remains useful, thusly... 1. Fast iteration and concepting; it can be preferable to use a VB setup to quickly put together basic forms which might need changing or updating at any time before a concept is finalised enough to become a polygonal model. 2. Transitional Forms and advantages over Booleans; For all those situations where boolean operations don't produce rounded or beveled transitions between operands VB can do that very easily and adjustably whilst not losing any parametricity. Uniquely they do this without needing to base the result on topology like bevel deformer on a boole would for example, and are therefore free from the artefacts that usually results in. 3. Combining with ReMesher to save modelling time; It is sometimes a lot easier to get a tricky base form started using the VB before remeshing it early on, and carrying on with poly modelling from there. 4. Infinite resolution; VB setups can be generated at any level of detail in a way that no other method can (OK, sculpting can too, I concede :) 5. Incorporation of splines, points, generators and matrices into modelling; No other systems within Cinema allow this as fully / directly as VB / VM. 6. Allows Noise-based and Field-based modelling; useful for producing the sort of meshes that are nigh on impossible to create any other way, such as a network of holes and tunnels in an Aero bar for example, or the internals of an igneous asteroid... 7. Unique Layer and folder-based hierarchy; with similarly unique elements like erode / dilate and multiple levels of smoothing. 8. Clouds / volume based elements; SDF is not the only mode here, we also have a fog mode which is how we can generate our own cloud and volumetric forms and the like... 9. Vector Mode; useful for spatially-based vector and field operations that can control and apply to forces. And there will be more I haven't mentioned - the Volume builder in Cinema is a lot wider in scope than first appears obvious, and with every improvement and iteration it gets it becomes more useful (see cache layers / transitions etc), and its less desirable behaviours get further minimised and improved upon. As primarily a trad modeller myself I don't use it as much as others do, but there are definitely times when it helps me and I call on it when other methods fall short, or would take an unreasonably long amount of time when time is an issue and deadlines are looming... CBR