Hi Retchem and welcome to Cruise-Chat!
We took the train ride a few years ago when we went to Panama. It was a nice excursion, but I don't remember whether it was handicap accessible. You may want to call the cruise line (or have your travel agent call) and ask them.
Usually the excursion booklet you receive from the cruiseline, or download off the internet, will mention restrictions. But I would call and make sure what accommodations are available.
Enjoy planning for your cruise! It is an exciting part of the total experience!