Download of JMRI 4.20 for Mac was on the developer's website when we last checked. We cannot confirm if there is a free download of this app available.
According to the results of the Google Safe Browsing check, the developer's site is safe. Despite this, we recommend checking the downloaded files with any free antivirus software. The JMRI installer is commonly called JMRI.2.12.dmg. This free software for Mac OS X was originally produced by jmri. The most popular version of JMRI for Mac is 2.1. The program is included in Design & Photo Tools.
From the developer:
This applications is made by some tools for model railroad computer control. It is written in java and it is independent of specific hardware systems. It is intended as a jumping-off point for hobbyists who want to control their layouts from a computer without having to create an entire system from nothing.
You may want to check out more software for Mac, such as Realsoft3D, FRS Coloring Book Volume 2 or FbxConverter, which might be related to JMRI.