The PROXmobil3 has two 1GB microSD cards by default - one facing sideways, and one facing down. The sideways one contains the rootFS, and the downways one contains the initFS.

The rootFS contains a minimal embedded install of PTXdist Linux 4.9, and is mounted read-only by default. It doesn't have much of anything beyond busybox and bash 4.3. It is difficult to remove, and it is not recommended to unless you broke something.

The initFS contains the fare reader program (at /init/efad/prgm/ProxNx) and associated startup functions, and is mounted at /init during boot. If you need additional storage capacity, it is recommended to replace this microSD card and bind mount necessary paths (e.g. /home) to the initFS microSD card.

Also on the initFS seem to be old versions of the CapMetro software? At /init/install/archive are files suffixed .X72, which seem to behave as normal zip files. A copy of ProxNx (the CapMetro software itself) is found upon extracting each one, along with associated configuration files.