XTrackCAD is available by a variety of methods:

Installing from your distribution, such as Ubuntu or Fedora

Installing the desired version from the XTrkCad release page; https://sourceforge.net/projects/xtrkcad-fork/files/XTrackCad/

Installing from the self-extracting archive.

Normally XTrkCAD is installed in /usr/... or /usr/local/.... If you install XTrackCAD into another directory, set the XTRKCADLIB environment variable to point to that directory.