just my own thoughts here (and it may be a given to you so sorry if im saying what you already know ) but certainly you are showing a still from a film seconds after the shots rang out , just after the limo passed below the underpass and was about to turn left on to the stemmons .at that point in time sure any car there , i mean there at that point and previous to that point would have been moved on by police . given that it let anyone in the car have close access to jfk . to be honest i cant recall if tilson said the car was always prior to jfks arrival and again shortly there after , but if he did that is a problem as the photo you posted shows no car in that area .could the car have pulled up in the minutes after the shooting ? .
update : ive just watched the video above to refresh my memory . and yest it appears as tho his is saying the car was always there . there is a sign in the way but even with that it does not look like there was a car there .so either he made this story up or he is confused and saw the car some minutes after the motorcade had passed by . regarding the train , there was a train in the railyard behind the fence , but we can see from film and photos that no train was on the overpass as jfk passed beneath .i will say tho that tilson was unruffled and kept cool .i am wondering why the bug didnt pull out a large still of the film frame you posted here and show it to tilson .