This is a hard rule to understand why. Apparently, someone was seriously injured or killed doing this somewhere at some time.
Most tank cars only have one vertical handhold at the side of the end platform. This makes it especially difficult to mount a moving tank car. Mounting moving equipment is now forbidden on many railroads. There have been instances where an employee misses a stirrup and gets their leg taken off. This could happen on any car, but is especially possible on tank cars and flat cars.
On the other hand, if you are walking alongside the car and it derails, you have a better chance of being crushed than if you were riding it.
Tank cars, probably more so than any other tank, have to be spotted in close-clearance locations. Loading racks; alongside buildings or storage tanks.
This means there are fewer opportunities to walk alongside the move.
The best alternative is to be ahead of the move and watch it as it approaches you. This, again, can be difficult. Loading/unloading racks can block the view, plus these tracks are often built on curves or even grades.
Les