Full instructions for installing JMRI on different platforms can be found here:
JMRI® requires that Java be installed:
As of JMRI 4.20, JMRI is only fully supported when running on the most current update possible to the two most recent Java Long Term Support (LTS) releases (8 or 11 as of July 2020) and will receive "best effort" support on the latest Java release (14 as of July 2020). Some problems encountered while using a Java version that is "best effort" may only be addressable by using a supported version.
The JMRI project recommends Java from AdoptOpenJDK or the operating system vendor unless using Windows 8 or older, in which case Java from Oracle is recommended. See the operating system-specific installation guidance for further details.
Once you have downloaded the JMRI® software and installed it, simply open the program in the usual manner for your operating system. Windows will show an icon on desktop (with default install options selected).
Double click the icon for the program to open. In this case we will open DecoderPro®
This dialog will appear briefly as the program is loading:
You should see this or a similar opening screen displayed after the program starts:
If this is the first time you run the program after you
installed the software, the Preferences screen will come up
automatically to allow you to set up the system for your
particular configuration. Also if you change your system
configuration, OR if you have your laptop away from the
railroad, but you want to play with JMRI there is a simulator
mode you may want to try.
[To be continued]
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