First of all, a little bit of Googling tells me that this "jmri" is Java Model Railroad Interface.
Second, it does not seem to exist as a Debian (Ubuntu) or as an RPi package, so if the OP did actually install it, it would have been from a "tarball" (or similar).
Third, this web site described the process (Yes, this is for Ubuntu and, presumably, x86, but the principles should still hold):
Fourth, yes, you will probably have to hunt around for the right binary tarball for the Pi, or (gasp!) compile it yourself from source.
Fifth, yes, as a general comment, it is true that when install stuff yourself (either from a binary tarball or via compiling from source), it is less easy to change your mind and uninstall it than it would be had you installed from a package manager. That's part of why package managers were invented...