Although it may be a tad expensive at the beginning, you may want to look into getting a highrail equipped trackhoe that has a mowing attachment where the digging bucket is normally mounted. I have seen my local highway department use these (minus the rail wheels) all the time, and I have seen at least one video that showed a rail mounted version. Plus, a highrail trackhoe can do all sorts of other MOW work just by interchanging the boom attachments. I have even seen non rail mounted trackhoes doing MOW work on railroads, although having a heavy tracked vertical driving on the ties and rails can damage them, especially if the rail is light.
Just my two cents.
Matt