You should contact the railroad company for that information.   Most railroad companies have a clearance diagram already part of their standard drawings for work in or around their tracks.  In general this clearance diagram addresses horizontal and vertical clearance as well as any excavation along the track.  While most of the clearances are for roadway bridges over tracks you should be able to get some information for your situation.  You should know that railroads are sometimes difficult to deal with so be patient.  Also note that AREA is just a code (i.e., a set of minima or maxima) and can be adopted differently by each railroad company differently just as AASHTO is adopted differently by each state.  Thus just because one rail co. does something one way don't assume another does the same.

Thanks for the information. I did look through the AREMA guidelines and did not see anything.

R.R. companies have a monopoly that is out of control. I understand safety is important, but everyone needs a check n balance.