I always thought that they were intended to snap off any offending tree/brush growth so the parts being shipped wouldn’t get snagged. Gues I’m wrong on that count…
As far as speed, I’m willing to bet these travel at a more moderate pace, and are lighter/shorter than a standard train so they can also stop faster… but then again, this is just my interpretation of how hazmat/airplane/rocket booster/etc trains run so I may be way off base.
Actually, these loads are just shipped in regular trains. There is no special speed restriction. The cars all say “Do Not Hump” on them - they don’t want them free rolling by themselves (as they would in a hump yard), but that’s because too severe a jar may crack the propellant, resulting in uneven burn when lit, and in an extreme case perhaps a vehicle loss (like the Challenger). I was in a class for shuttle operations one time and someone asked what “Do Not Hump” means. The instructor said that it meant the cars should just be handled carefully. I quickly explained the actual meaning of the label.