I saw the movie a couple of years ago. The bikes were BSA B40's or B44's with their POS Prince of Darkness electrical systems that lasted until Albuquerque.
They were then traded in for some lightweight Japanese machines.
I made the same trip myself over the winter of 2009/2010 on a Kawasaki KLR650 but stayed off the tracks at least in the USA.
Did my rail riding in Guatemala where I ended up being the last ever passenger before the rails were stolen.
In the movie they sailed home...I rode
15,000 miles round trip, a fantastic experience which included two weeks in Cuba.
TH