Azizi, in several threads, I have disputed the notion of simple code songs associated with the underground railway or with escape. To believe in them, one would also have to believe that slaves were subnormal in intelligence and had no means of communication. Much depended upon passing on information about how to contact people who would help and how to find safe houses. Explicit information passed by word of mouth was necessary.
Slave narratives from those who escaped tell of such help, or tell of careful, solitary escapes by one who was smart enough to know when to move and where and when to hide. A successful escapee had to be brave, intelligent and observant.
There are threads on this subject, which would be better discussed there.

At the time these gospel songs were written, the term "holy roller," to a member of a more organized church, could simply mean someone with evangelical leanings, or who went to the tent gatherings set up by a traveling preacher and his entourage. Scarborough, if she was like my grandparents and I think she was, applied the term in this way. People who worshipped noisily and without discipline were avoided.
Gospel train songs succeeded the ship and chariot songs in part. Scarborough comments that the Negro overlooked the automobile, but although poor whites and blacks might own a rickety car or truck, the train was the premier mode of travel almost up to WW2. If one was really going somewhere, they put on their best clothes and boarded the train.