A machine or crane controller is essentially a COSS for on track plant.
As every work group needs a COSS, every on track crane or RoadRailer requires a crane controller. You're the eyes and ears of the operator, control the moves and also determine what is and what is not acceptable for the crane to move.
To get into such a role, you would need several competencies. You will need a PTS, a Track Induction, be a Controller of Site Safety and then go through the Crane/Machine Controller course. This is NOT an entry level job that you can go straight into, nor will any of the big Rail Construction companies (Network Rail, Balfour Beatty, Amey Colas etc) recruit off the streets for such roles.
Your best opportunity to get into the positions that you enquire about is to get sponsored by a contractor such as McGinley, SkyBlue, Vital, Cleshsar to name a few, prove yourself on track then ask to attain the further training.