Trying to draw an argument that since there's so many more road fatalities than rail fatalities that trains are super safe ignores a lot of variables: how many people die per x amount of miles traveled, mixing road accidents by amateur drivers (most of us, myself included) vs. professional drivers, etc.
I couldn't easily find a non-biased source for deaths per x unit of vehicle miles traveled, at least from a non-biased source (there was something from the Cato Institute, but it was in a very anti-rail policy brief, so I didn't want to rely on it.) The best calculation I could do from the non-biased information I could find was to compare the highway bus fatality count compared to highway bus total miles traveled vs. rail, passenger "train accidents" and potentially "rail, other" deaths compared to Intercity/Amtrak and Commuter rail miles traveled. (I'm not sure if "rail, other" is a category to include for deaths, but it does exclude trespasser and rail crossing deaths. I'm also not sure if rail, passenger includes commuter rail, but it's not specifically called out elsewhere so I'm assuming it's considered part of rail passenger, whereas transit isn't.)
According to
Table 1-40 from the National Transportation Statistics, published by the Bureau of Transportation Statistics, in 2014 (the latest year with full data) there were 339,177 million passenger miles traveled by highway bus, versus 18,275 million passenger miles traveled by Amtrak, intercity, and commuter rail in 2014.
Table 2-1 from the same source states that 3 people died from train accidents for passenger rail in 2014, and 12 died after including in "rail, other." That same year, 44 bus passengers died while traveling.
Even on the low end, assuming none of the "rail, other" deaths were passenger deaths, there was one death per 6092 million passenger miles by rail, vs one passenger death per 7709 million passenger miles by bus. To me, that signals that bus transportation is likely just as safe, if not safer, than rail transportation.
There's plenty of reasons to oppose bus-bridging part of the SWC's route, but safety isn't one of them. Bus transportation is just as safe as rail transportation.