Miss Adams: Yes, sir; going down the stairs toward the back, I was running. I ran to the railroad tracks.
You are aware, are you not, that there is a railroad track directly next to the TSBD annex which goes behind the loading dock?
So, Adams and Styles, after leaving the loading dock were always going to be running "to the railroad tracks".
The decision to run there could have easily be made just after they left the loading dock. And it wouldn't have been a difficult decision to make, as they knew that the shots were fired at the motorcade as it passed the grassy knoll.
There wouldn't have been much point in running in the other direction, would it now?