It’s very easy for a low vehicle like a sedan or coupe to get stuck on the tracks, They have a low ground clearance and if it wasn’t perfectly between the rails it likely High-centered and got stuck and even if somehow it did manage to fit between the rails it would be very difficult to keep it straight going backwards.
Never drive on railroad tracks for any reason.

I had the same question. It’s buried in the next paragraph.

In this case, however, it appears the Tesla was able to drive along the tracks for a while before getting stuck.