Hi,
Neither Forest Pack or RailClone have a circle packing algorithm which is what you'd require to do this really well, however you might be able to get close enough in Forest Pack. I've attached a sample file to illustrate a possible approach.
- In this file I'm using just one cylinder with randomised scale.
- Collisions are enabled and left at their defaults. Preview in Viewport is enabled to see the results.
- Scale is randomised, but I've edited the probability to reduce the number of large cylinders which otherwise tend to dominate.

- Distribution map is set to solid. You can play with the size of the map to get the desired results.

I hope that helps.
Paul