Assuming he can legally possess it in RI, to transport it, put it in a locked gun case, and keep ammo in a seperate locked container. Throw the locked cases in his suitcase and take it on the train with him. Unless the train has a policy against firearms, (make sure to check this out). If the train company has rules against it, the only option would be to either drive, or have an FFL ship it to another FFL near you in RI.
For RI firearms laws:
http://www.packing.org/states.jsp
(I think I covered everything, maybe someone will correct me, or think of something I forgot)