The tracks in the concrete appear before the railcar wheels rolled over it.
The idea of Grampa suffering a "kidney blowout" is medically impossible. His bladder would have been the organ that would have ruptured from holding urine in for too long, since the bladder is the organ that retains the urine before it gets expelled by urination (the kidneys just filter what the body has consumed).
In this episode, Grampa has two kidneys, but in Season 2 episode "Old Money", Grampa told Bea that he had one (unless he meant one that was still functioning).
When Homer was parallel-parking the railcar, he was out of proportion compared to the cars on either side of him. Oddly, he barely comes up to the height of the boot on the closest car.
The Frenchman says he stole his accordion from a blind monkey, but what he's actually playing is a concertina. An accordion has a full keyboard on it, is boxy rather than circular, and wouldn't be held easily with two hands (accordions have straps on them worn on the body that keep it in place).
Homer is stated to still have both kidneys in later episodes, and when he is seen shirtless in "Alone Again, Natura-Diddily" there is no kidney-shaped scar on his body.